Positive active material for nonaqueous electrolyte secondary battery, method of manufacturing the positive active material, electrode for nonaqueous electrolyte secondary battery, nonaqueous electrolyte secondary battery and method of manufacturing the secondary battery

ABSTRACT

An object of the present invention is to provide a positive active material for a nonaqueous electrolyte secondary battery which has a large discharge capacity and is superior in charge-discharge cycle performance, initial efficiency and high rate discharge performance, and a nonaqueous electrolyte secondary battery using the positive active material. The present invention pertains to a positive active material for a nonaqueous electrolyte secondary battery containing a lithium transition metal composite oxide which has a crystal structure of an α-NaFeO 2  type, is represented by a compositional formula Li 1+α Me 1−α O 2  (Me is a transition metal element including Co, Ni and Mn, α&gt;0), and has a molar ratio Li/Me of Li to the transition metal element Me of 1.2 to 1.6, wherein a molar ratio Co/Me of Co in the transition metal element Me is 0.02 to 0.23, a molar ratio Mn/Me of Mn in the transition metal element Me is 0.62 to 0.72, and the lithium transition metal composite oxide is observed as a single phase attributed to a space group R3-m on an X-ray diffraction chart when it is electrochemically oxidized up to a potential of 5.0 V (vs. Li/Li + ).

TECHNICAL FIELD

The present invention relates to a positive active material for anonaqueous electrolyte secondary battery and a nonaqueous electrolytesecondary battery using the same.

BACKGROUND ART

Conventionally, LiCoO₂ is mainly used as a positive active material fora nonaqueous electrolyte secondary battery. However, the dischargecapacity of LiCoO₂ is about 120 to 130 mAh/g.

A solid solution of LiCoO₂ and another compound is known as a materialof a positive active material for a nonaqueous electrolyte secondarybattery. Li [Co_(1−2x)Ni_(x)Mn_(x)] O₂ (0<x≤½), which has a crystalstructure of an α-NaFeO₂ type and is a solid solution of threecomponents, LiCoO₂, LiNiO₂ and LiMnO₂, has been presented in 2001.LiNi_(1/2)Mn_(1/2)O₂ or LiCo_(1/3)Ni_(1/3)Mn_(1/3)O₂ as an example ofthe solid solution has a discharge capacity of 150 to 180 mAh/g and isalso superior in charge-discharge cycle performance.

For the above-mentioned so-called “LiMeO₂ type” active material, theso-called “lithium excess type” active material, in which acompositional ratio Li/Me of lithium (Li) to a transition metal (Me) ismore than 1 and for example Li/Me is 1.25 to 1.6, is known. Acompositional formula of such a material can be denoted byLi_(1+α)Me_(1−α)O₂ (α>0). Here, when the compositional ratio Li/Me oflithium (Li) to a transition metal (Me) is denoted by β, β=(1+α)/(1−α),and thus α=0.2 if Li/Me is 1.5, for example.

In Patent Document 1, an active material, which is a kind of such anactive material and can be represented as a solid solution of threecomponents of Li[Li_(1/3)Mn_(2/3)]O₂, LiNi_(1/2)Mn_(1/2)O₂ and LiCoO₂,is described. Further, as a method of manufacturing a battery using theabove-mentioned active material, it is described that by providing aproduction step in which charge at least reaching a region where apotential change is relatively flat, occurring within a positiveelectrode potential range of more than 4.3 V (vs. Li/Li⁺) and 4.8 V (vs.Li/Li⁺) or less, is performed, it is possible to manufacture a batterywhich can achieve a discharge capacity of 177 mAh/g or more even whenemploying a charge method in which a maximum achieved potential of apositive electrode at the time of charge is 4.3 V (vs. Li/Li⁺) or less.

Such a so-called “lithium-excess type” positive active material has aproblem that charge-discharge cycle performance and high rate dischargeperformance are not sufficient. Further, as described above, when in atleast the first charge, charge is performed up to a relatively highpotential more than 4.3 V, particularly up to a potential of 4.4 V ormore, there is a feature of achieving a high discharge capacity, butinitial charge-discharge efficiency (hereinafter, referred to as initialefficiency) in this case is not adequately high. Moreover, in PatentDocument 1, the stability of the crystal structure, the oxygen positionparameter, the specific surface area, and the tapped density are notdescribed.

In Patent Document 2, it is described that a lithium-containing metalcomposite oxide of a layered rock salt type, an oxygen positionparameter and a distance between lithium and oxygen relate to an initialdischarge capacity and charge-discharge cycle performance. However, itis not described how the oxygen position parameter affects the high ratedischarge performance.

In Patent Documents 3 and 4, an active material for a lithium secondarybattery of the general formula xLiMO₂.(1−x)Li₂M′O₃ (0<x<1) is described,and it is also described that M is at least one selected from Mn, Co andNi, and Mn is selected for M′, and it is shown that the active materialcontaining enriched Li stabilizes a crystal structure, and by using theactive material, a lithium secondary battery having a large dischargecapacity is attained, but the stability of the crystal structure inbeing electrochemically oxidized to a high potential is not clear, andimprovements in charge-discharge cycle performance, initial efficiencyand high rate discharge performance are not described. Also, in thesePatent Documents, an active material in which the content of Mn is largeand the content of Co is small is not specifically described, and theoxygen position parameter, the specific surface area, and the tappeddensity are not also described.

In Patent Document 5, an active material for a lithium secondary batteryof the general formula Li_(1+x)Ni_(α)Mn_(β)A_(γ)O₂ (x is 0 to 0.2, α is0.1 to 0.5, β is 0.4 to 0.6, and γ is 0 to 0.1) is described, and it isalso described that Co is selected for A, and it is shown that by usingthe active material having the above composition and containing enrichedLi, which is produced by a specific method, a lithium secondary batteryhaving a large discharge capacity is attained, but improvements incharge-discharge cycle performance, initial efficiency and high ratedischarge performance are not described. Further, in Patent Document 5,the invention of an active material in which a molar ratio of Li to atransition metal element is 0.2 or more and the content of Mn in thetransition metal element is more than 0.6 is not described, and thestability of the crystal structure, the oxygen position parameter, thespecific surface area, and the tapped density are not also described.

In Patent Document 6, described is “A lithium battery, wherein when atleast one transition metal selected from Groups 7A and 8A of theperiodic table is denoted by Me, a transition metal different from theMe is denoted by Mt, and at least one element selected from the groupconsisting of Mt, Na, K, Rb, Cs, Al, Ga, In, Tl, B, Mg, Ca, Sr, Ba andPb is denoted by A, the battery includes a positive active materialcomprising a composite oxide having the composition represented byLi_(X)Me_(Y)A_((1−Y))O_((1+X)) (1.3≤X≤2.5, 0.5≤Y≤0.999), and a hexagonalcrystal structure” (claim 1), and it is shown that the positive activematerial containing enriched Li stabilizes a crystal structure, and byusing the positive active material, a lithium secondary battery having ahigh energy density is attained, but the stability of the crystalstructure in being electrochemically oxidized to a high potential is notclear, and improvements in charge-discharge cycle performance, initialefficiency and high rate discharge performance are not described.Further, in Patent Document 6, an active material in which x is 1.3, Meis Mn, A is Co, the content of Mn is large and the content of Co issmall is described, but it is not specifically described that Co and Niare selected as A, and the oxygen position parameter, the specificsurface area, and the tapped density are not also described.

In Patent Document 7, described is the invention of “A positiveelectrode material for a nonaqueous electrolyte secondary battery usinga lithium manganese nickel cobalt oxide comprising lithium, manganese,nickel, cobalt and oxygen, wherein the lithium manganese nickel cobaltoxide has a layered structure and is represented byLi[Li_([(1−2x−y)/3])Ni_(x)Co_(y)Mn_([(2−x−2y)/3])]O₂, and x and ysatisfy 0.2<x<0.5, 0<y<0.2, and 1<2x+y” (claim 1), and it is shown thatby using the positive electrode material, cycle characteristics areimproved, but improvements in initial efficiency and high rate dischargeperformance are not described. Further, as an example,Li_(1.15)Ni_(0.25)Co_(0.05)Mn_(0.55)O₂, a lithium manganese nickelcobalt oxide in which a molar ratio Li/Me of Li to all transition metalelements Me is 1.353, a molar ratio Co/Me is 0.059, and a molar ratioMn/Me is 0.647, is described (Example 4), but the stability of thecrystal structure of the positive electrode material, the oxygenposition parameter, the specific surface area, and the tapped densityare not described.

In Patent Document 8, described is “A positive active materialcontaining a lithium composite oxide represented by the followingchemical formula: [Chem. 1]Li_(1+a)[Mn_(b)Co_(c)Ni_((1−b−c))]_((1−a))O_((2−d)), wherein a, b, c andd satisfy 0<a<0.25, 0.5≤b<0.7, 0≤c<(1−b), and −0.1≤d≤0.2” (claim 1), andit is shown that by using the positive active material, a largedischarge capacity and good cycle characteristics can be realized, andcharge-discharge efficiency is also shown, but the invention is notintended to improve the charge-discharge efficiency, and an improvementin high rate discharge performance is not described. Further, as anexample, lithium composite oxides in which a molar ratio Li/Me of Li toall transition metal elements Me is 1.30 are described (Example 1-3,Examples 2-2 to 2-8, Examples 3-1 and 3-2), but these composite oxidesare synthesized by using “a solid state method”, and most of thecomposite oxides contain less Mn. As only one active material containingMn in a large amount, Li_(1.13)[Mn_(0.65)Co_(0.20)Ni_(0.15)]_(0.87)O₂ isshown, but the stability of the crystal structure of the positive activematerial, the oxygen position parameter, the specific surface area, andthe tapped density are not described.

On the other hand, with respect to lithium transition metal compositeoxides composed of Li and transition metal elements (Co, Ni, Mn, etc.),active materials in which the specific surface area and the tappeddensity are increased are known (refer to e.g., Patent Documents 9 and10).

In Patent Document 9, described is the invention of “A positive activematerial having a laminar crystal structure, wherein a sequence of alithium element and an oxygen element composing an oxide composed ofcrystal particles of the oxide containing at least three transitionmetal elements is a cubic structure, and a specific surface area is 0.9to 2.5 m²/g, and a tapped density is 1.8 to 2.5 g/cm³” (claim 1), and itis shown that in accordance with this invention, a lithium secondarybattery having high initial charge-discharge efficiency (initialefficiency) and excellent durability of a charge-discharge cycle can beobtained. Further, in Patent Document 9, described is “A positive activematerial for a lithium secondary battery represented by the generalformula Li[LiqCoxNiyMnz]O₂, in which q satisfies −0.2≤q≤0.2,0.8≤1+q≤1.2, X satisfies 0.1<X≤0.6, Y satisfies 0.1<Y≤0.6, Z satisfies0.2<Z≤0.6, and X, Y and Z satisfy 0.7≤X+Y+Z≤1.2” (claim 2), but since apositive active material, in which a molar ratio of Li to the transitionmetal element is 1.2 or more and a molar ratio of Mn in the transitionmetal element is 0.625 or more, is not specifically described, theinitial efficiency and the charge-discharge cycle performance of thepositive active material having such composition cannot be predicted.Further, the stability of the crystal structure composing the positiveactive material and the oxygen position parameter are not described.

In Patent Document 10, described is the invention of “A lithium nickelmanganese cobalt composite oxide for a lithium secondary batterypositive active material represented by the following general formula(1):Li_(x)Ni_(1−y−z)Mn_(y)Co_(z)O₂  (1)

in which x satisfies 0.9≤x≤1.3, y satisfies 0<y<1.0, z satisfies0<z<1.0, and y and z satisfy y+z<1, wherein an average particle size is5 to 40 μm, a BET specific surface area is 5 to 25 m²/g, and a tappeddensity is 1.70 g/ml or more” (claim 1), and it is shown that inaccordance with this invention, a lithium secondary battery having highinitial efficiency and excellent loading characteristics (high ratedischarge performance) can be obtained. However, since a positive activematerial in which x is 1.2 or more and y is 0.625 or more is notspecifically described in Patent Document 10, the initial efficiency andthe high rate discharge performance of the positive active materialhaving such composition cannot be predicted. Further, the stability ofthe crystal structure composing the positive active material and theoxygen position parameter are not described.

In improvement in high rate discharge performance of the lithiumtransition metal composite oxide in which lithium is enriched, partialfluorination of a part of oxygen (Non-patent Document 1) and a surfacecoating technology (Non-patent Document 2) are proposed. However, all ofthese are technologies expecting use at 4.5 V or more as a positiveelectrode charge potential corresponding to a potential region ofdecomposition of an electrolyte solution, and are not technologiesintended to improve high rate discharge performance at the time when thepositive electrode charge potential is changed to a potential lower than4.5 V, for example, 4.3 V, after initial formation to use a battery.

Moreover, the so-called “lithium-excess type” positive active materialhas a problem that oxygen gas is generated during charge (refer to e.g.,Patent Documents 11, 12, and Non-patent Documents 3, 4).

In Patent Document 11, described is the invention of “A method ofmanufacturing an electrochemical element comprising the step of chargingan electrode active material having a plateau potential, at which gas isgenerated in a charge range, to the plateau potential or more; and thestep of removing the gas” (claim 1), “The manufacturing method accordingto any one of claims 1 to 3, wherein the positive active material has aplateau potential of 4.4 to 4.8 V” (claim 4), “The manufacturing methodaccording to claim 1, wherein the gas is oxygen (O₂) gas” (claim 5), “Anelectrochemical element, wherein an electrode active material having aplateau potential at which gas is generated in a charge range is chargedto the plateau potential or more, and then the gas is removed” (claim6), “The electrochemical element according to claim 6, wherein theelectrode active material has a plateau potential of 4.4 to 4.8 V”(claim 7), and “The electrochemical element according to claim 8,wherein after charging to the plateau potential or more and removinggas, a discharge capacity of the electrode active material ranges from100 mAh/g to 280 mAh/g in a voltage range of 3.0 to 4.4 V” (claim 10).

Further, when as the above electrode active material, a chemical formula1 “a solid solution of XLi(Li_(1/3)M_(2/3))O₂+YLiM′O₂ in which M is oneor more elements selected from metals having an oxidation number of 4+,M′ is one or more elements selected from transition metals, and X and Ysatisfy 0<X<1, 0<Y<1, and X+Y=1” (claim 2, claim 8, and paragraph[0024]) is used, “When being charged to an oxidation-reduction potentialof M′ or more, Li is extracted and simultaneously oxygen is alsodetached in order to have a balance between oxidation and reduction.Accordingly, the electrode active material has a plateau potential”(paragraph [0025]), “The compound of the chemical formula 1 is preferredsince the electrode active material functions stably as an electrodeactive material in a charge-discharge cycle after the electrode activematerial is charged to a charge voltage (4.4 to 4.8 V) of a plateaupotential or more and the gas removal step is performed” (paragraph[0026]), and “Preferably, M is one or more elements selected from Mn, Snand Ti metals, and M′ is one or more elements selected from Ni, Mn, Coand Cr metals” (paragraph [0027]) are described.

Moreover, in Patent Document 11, “When a battery is configured by amethod in which the active material is charged to a plateau potential ormore once or more and then the gas removal step is performed accordingto the present invention, even though the active material is charged toa plateau potential or more continuously, a battery having a highcapacity is configured, and a problem of a battery due to gas generationcan also be solved. That is, After charging to a plateau potential ormore, gas is not generated in charge in the subsequent cycles, and aplateau range disappears (refer to FIG. 4) (paragraph [0022]) isdescribed, and as Example 4, it is shown that in the case whereLi(Li_(0.2)Ni_(0.2)Mn_(0.6))O₂(3/5[Li(Li_(1/3)Mn_(2/3))O₂]+2/5[LiNi_(1/2)Mn_(1/2)]O₂)is used as a positive active material (paragraph [0048]), and “chargedto 4.8 V in a first cycle, and charged to 4.4 V in a second cycle”(paragraph [0060]), a battery having a high capacity can be obtained(refer to FIG. 5). However, it is suggested that as described inComparative Examples 5 and 6, oxygen gas is not generated when chargingup to 4.25 V or 4.4 V that is a plateau potential or less, but only abattery having a low discharge capacity can be obtained (paragraph[0056], FIG. 1, paragraph [0057], and FIG. 2), and therefore it cannotbe said that a positive active material, which does not generate oxygengas even when charging up to a voltage higher than a plateau potential,is shown.

In Non-patent Documents 3 and 4, it is shown that whenLi[Ni_(x)Li_((1/3−2x/3))Mn_((2/3−x/3))]O₂ is used as a positive activematerial, oxygen gas is generated at a charge voltage (4.5 V to 4.7 V)that is a plateau potential or more (left column line 4 to right columnline 2 in page A818 in Non-patent Document 3, left column line 9 in pageA785 to right column line 4 in page A788 in Non-patent Document 4), butit is not shown that oxygen gas is not generated when charging up to ahigh voltage that is a plateau potential or more.

In Patent Document 12, described is the invention of “A nonaqueous-type(lithium ion) secondary battery formed by winding or layer stacking apositive electrode plate, in which a current collector is provided withan active material layer capable of intercalation/deintercalationlithium ions thereon, and a negative electrode plate with a separatorsandwiched between the electrode plates to form an electrode group, andhousing the electrode group in a case hermetically together with anonaqueous electrolyte, wherein an active material to be charged at 4.3V or less on the Li/Li⁺ basis and a substance to generate oxygen gas atthe time of overcharge exist on the positive electrode plate” (claim 1),and “The lithium ion secondary battery according to claim 1, wherein anactive material represented byLi[(Ni_(0.5)Mn_(0.5))_(x)Co_(y)(Li_(1/3)Mn_(1/3))_(z)]O₂ (x+y+z=1, z>0)or LiαNiβMnγO₂ (α is 1.1 or more, β:γ=1:1) is used as the substance togenerate oxygen gas at the time of overcharge” (claim 2), and it isdescribed that the lithium-excess transition metal composite oxides(Examples 1 to 3 and 7: Li_(1.2)Ni_(0.4)Mn_(0.4)O₂, Examples 4 to 6:Li[(Ni_(0.5)Mn_(0.5))_(1/12)Co_(1/4)(Li_(1/3)Mn_(2/3)1/3)]O₂ (x= 5/12,y=¼, z=⅓)) easily generate oxygen gas at the time of overcharge incomparison with the lithium transition metal composite oxide (notlithium-excess) (Comparative Examples 1: LiCoO₂, Comparative Example 2:LiNi_(0.5)Mn_(0.5)O₂, Comparative Example 3:Li(N_(1/3)Mn_(1/3)Co_(1/3))O₂) (paragraph [0064]), and it is describedthat in the invention described in Patent Document 12, on the contrary,utilizing the above-mentioned property of the lithium-excess transitionmetal composite oxide, “By gas generation at the time of overcharge,since the positive active material layer is detached from the currentcollector, the positive electrode plate is detached from the separator,or positive electrode layer inside is split, it is possible to cut outcharge and prevent decomposition of the electrolyte solution,decomposition of the positive active material and short-circuit due todeposit of Li to the negative electrode side” (paragraph [0010]).

Moreover, since it is described in Patent Document 12 that “In thelithium ion secondary battery of the present invention, as the positiveactive material generating oxygen gas at the time of overcharge, it ispreferred to use a lithium-excess positive active materialLi[(Ni_(0.5)Mn_(0.5))_(x)Co_(y)(Li_(1/3)Mn_(1/3))_(z)]O₂ (whereinx+y+z=1, z>0) or an active material represented by LiαNiβMnγO₂ (α is 1.1or more, β:γ=1:1). When the above active material is used, oxygen gas isgenerated at about 4.5 V on the Li/Li⁺ basis, and thereby, a distancebetween the positive electrode and the negative electrode can beincreased”, the positive active material not generating oxygen gas whencharging up to 4.5 V or more is not shown.

On the other hand, since oxygen gas generated from the positive activematerial causes failures such as oxidation of a solvent constituting anelectrolyte of a nonaqueous electrolyte secondary battery (lithiumsecondary battery) and heating of the battery, a positive electrode(positive active material) for a nonaqueous electrolyte secondarybattery in which oxygen gas generation at the time of overcharge orhigh-temperature is suppressed is also developed (refer to e.g., PatentDocuments 13 and 14).

With respect to the invention described in Patent Document 13, in claim1, difficulty of generation of oxygen gas of a lithium-containingcomposite oxide (lithium transition metal composite oxide) is specifiedas “a local maximum value of oxygen generation peak in gaschromatography-mass spectrometry measurement of the composite oxide” and“a range of 330 to 370° C.”, and it is described that “In GC/MSmeasurement, a temperature of a positive composite is raised at a rateof 10° C./min from room temperature to 500° C., and behavior of oxygengeneration was observed. Here, the obtained oxygen generation spectrum(A) is shown in FIG. 3. As is apparent from FIG. 3, in the spectrum (A),a local maximal value of an oxygen generation peak is positioned at aside of temperature higher than 350° C. From this, it is evident thatthe positive active material of the present invention is hardlydecomposed while generating oxygen, and is extremely superior instability even when being exposed to high-temperature in overchargeregion of a battery voltage of 4.7 V” (paragraph [0041]).

However, in Patent Document 13, only the positive active material“represented by the general formula: Li_(z)Co_(1−x−y)Mg_(x)M_(y)O₂,wherein an element M in the general formula is at least one selectedfrom the group consisting of Al, Ti, Sr, Mn, Ni and Ca, and x, y and zin the general formula satisfy 0≤z≤1.03, 0.005≤x≤0.1, and 0.001≤y≤0.03”is specifically described (claims 2 and 3), and the “lithium-excesstype” positive active material not generating oxygen gas in anovercharge region is not shown.

In Patent Document 14, it is shown that release of oxygen from apositive electrode at the time of high-temperature is suppressed bymixing an oxygen-storing material with a positive active material orattaching the oxygen-storing material to the positive active material(claim 1, paragraphs [0005], [0056]), and it is described that “Peaktemperatures of oxygen detachment of the positive electrodes (samples 1to 4) having a Ce oxide or a Ce—Zr oxide as an oxygen-storing materialare all 300° C. or higher, and the peak temperatures were significantlyincreased relative to that in the sample 5 not having an oxygen-storingmaterial. This shows that in the samples 1 to 4, a phenomenon ofreleasing oxygen from the positive electrode is suppressed better (tohigher temperature) than the phenomenon in the sample 5” (paragraph[0057]), but the positive active material (lithium transition metalcomposite oxide) not generating oxygen gas at a high-temperature is notshown.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A-2010-086690-   Patent Document 2: JP-A-2002-124261-   Patent Document 3: U.S. Pat. No. 6,677,082 Specification-   Patent Document 4: U.S. Pat. No. 7,135,252 Specification-   Patent Document 5: U.S. Pat. No. 7,314,684 Specification-   Patent Document 6: JP-A-10-106543-   Patent Document 7: JP-A-2005-100947-   Patent Document 8: JP-A-2007-220630-   Patent Document 9: JP-A-2006-93067-   Patent Document 10: JP-A-2009-205893-   Patent Document 11: JP-A-2009-505367-   Patent Document 12: JP-A-2008-226693-   Patent Document 13: JP-A-2004-220952-   Patent Document 14: JP-A-2006-114256

Non-Patent Documents

-   Non-patent Document 1: Thackeray et al., 155(4), 269-275 (2008)-   Non-patent Document 2: Kang et al., JPS, 146, 654-657 (2005)-   Non-patent Document 3: Journal of The Electrochemical Society,    149 (7) A815-A822 (2002)-   Non-patent Document 4: Journal of The Electrochemical Society,    149 (6) A778-A791 (2002)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is a first object of the present invention to provide a positiveactive material for a nonaqueous electrolyte secondary battery having alarge discharge capacity and excellent charge-discharge cycleperformance, and a nonaqueous electrolyte secondary battery using thepositive active material.

It is a second object of the present invention to provide a positiveactive material for a nonaqueous electrolyte secondary battery having alarge discharge capacity and excellent initial efficiency, and anonaqueous electrolyte secondary battery using the positive activematerial.

It is a third object of the present invention to provide a positiveactive material for a nonaqueous electrolyte secondary battery having alarge discharge capacity and excellent high rate discharge performance,and a nonaqueous electrolyte secondary battery using the positive activematerial.

It is a fourth object of the present invention to provide a positiveactive material which does not generate oxygen gas from a lithiumtransition metal composite oxide even when a battery is charged up to ahigh voltage, and has a large discharge capacity and has a largedischarge capacity particularly even when a charge method, in which amaximum achieved potential of a positive electrode at the time of chargeis lower than 4.4 (vs. Li/Li⁺), is employed, and to provide a nonaqueouselectrolyte secondary battery using the positive active material and amethod of manufacturing the nonaqueous electrolyte secondary battery.

Means for Solving the Problems

A constitution and an operation effect of the present invention will bedescribed including technical thought. However, an operation mechanismincludes presumption, and its right and wrong does not limit the presentinvention. In addition, the present invention may be carried out inother various forms without departing from the spirit and main features.Therefore, embodiments and examples described later are merelyexemplifications in all respects and are not to be construed to limitthe scope of the present invention. Moreover, variations andmodifications belonging to an equivalent scope of the claims are allwithin the scope of the present invention.

In the present invention, the following means are employed in order tosolve the above-mentioned problems.

(1) A positive active material for a nonaqueous electrolyte secondarybattery containing a lithium transition metal composite oxide which hasa crystal structure of an α-NaFeO₂ type, is represented by acompositional formula Li_(1+α)Me_(1−α)O₂ (Me is a transition metalelement including Co, Ni and Mn, α>0), and has a molar ratio Li/Me of Lito the transition metal element Me of 1.2 to 1.6, wherein a molar ratioCo/Me of Co in the transition metal element Me is 0.02 to 0.23, a molarratio Mn/Me of Mn in the transition metal element Me is 0.62 to 0.72,and the lithium transition metal composite oxide is observed as a singlephase attributed to a space group R3-m on an X-ray diffraction chartwhen it is electrochemically oxidized up to a potential of 5.0 V (vs.Li/Li⁺).

(2) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (1), wherein in the lithium transitionmetal composite oxide, a molar ratio Li/Me of Li to the transition metalelement Me is 1.25 to 1.40.

(3) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (1), wherein in the lithium transitionmetal composite oxide, an oxygen position parameter, determined bycrystal structure analysis by a Rietveld method at the time of using aspace group R3-m as a crystal structure model based on an X-raydiffraction pattern in a state of a discharge end, is 0.260 or less.

(4) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (2), wherein a BET specific surface areais 0.88 m²/g or more.

(5) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (4), wherein a tapped density is 1.25g/cm³ or more.

(6) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (2), wherein a molar ratio Li/Me of Li toall transition metal elements Me is 1.250 to 1.350, a molar ratio Co/Meis 0.040 to 0.195, and a molar ratio Mn/Me is 0.625 to 0.707.

(7) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (1), wherein a ratio between thediffraction peak intensity I₍₀₀₃₎ of (003) line and the diffraction peakintensity I₍₁₁₄₎ of (114) line based on X-ray diffraction measurementbefore charge-discharge satisfies I₍₀₀₃₎/I₍₁₁₄₎≥1.20.

(8) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (1), wherein a molar ratio Li/Me of Li tothe transition metal element Me is 1.25 to 1.40, and oxygen gas is notgenerated from the lithium transition metal composite oxide when chargeis performed up to any potential within the range of 4.5 to 4.6 V (vs.Li/Li⁺) as a maximum achieved potential of a positive electrode.

(9) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (1), wherein a molar ratio Li/Me of Li tothe transition metal element Me is 1.25 to 1.40, and a volume ratio ofoxygen to the total amount of nitrogen and oxygen, respectivelycontained in gas in a battery, is 0.20 to 0.25 when charge is performedup to any potential within the range of 4.5 to 4.6 V (vs. Li/Li⁺) as amaximum achieved potential of a positive electrode.

(10) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (8), wherein oxygen gas is not generatedfrom the lithium transition metal composite oxide when charge isperformed up to any potential within the range of 4.55 to 4.6 V (vs.Li/Li⁺) as a maximum achieved potential of a positive electrode.

(11) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (8), wherein the charge is a charge inthe initial charge-discharge.

(12) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (8), wherein the positive active materialhas a plateau potential in a charge range, and any potential within therange of 4.5 to 4.6 V (vs. Li/Li⁺) is the plateau potential or more.

(13) The positive active material for a nonaqueous electrolyte secondarybattery according to the above (1), wherein the lithium transition metalcomposite oxide is obtained by mixing/sintering a coprecipitatedprecursor of compounds of the transition metal elements including Co, Niand Mn, and a lithium compound.

(14) A method of manufacturing the positive active material for anonaqueous electrolyte secondary battery according to any one of theabove (1) to (13), comprising the steps of coprecipitating compounds oftransition metal elements including Co, Ni and Mn in a solution toproduce a coprecipitated precursor; and mixing/sintering thecoprecipitated precursor and a lithium compound.

(15) The method of manufacturing a positive active material for anonaqueous electrolyte secondary battery according to the above (14),wherein a pH in the step of coprecipitating compounds of transitionmetal elements including Co, Ni and Mn in a solution to produce acoprecipitated precursor is 8.5 to 11.0.

(16) The method of manufacturing a positive active material for anonaqueous electrolyte secondary battery according to the above (14),wherein a sintering temperature in the step of mixing/sintering thecoprecipitated precursor and a lithium compound is 800 to 940° C.

(17) An electrode for a nonaqueous electrolyte secondary batterycontaining the positive active material for a nonaqueous electrolytesecondary battery according to any one of the above (1) to (13).

(18) A nonaqueous electrolyte secondary battery including the electrodefor a nonaqueous electrolyte secondary battery according to the above(17).

(19) The nonaqueous electrolyte secondary battery according to the aboveparagraph (18), wherein a charge method, in which a maximum achievedpotential of a positive electrode at the time of charge is lower than4.4 V (vs. Li/Li⁺), is employed at the time of use.

(20) A method of manufacturing a nonaqueous electrolyte secondarybattery, wherein in a method of manufacturing a lithium secondarybattery in which a positive active material containing a lithiumtransition metal composite oxide is used and a step including initialcharge-discharge is performed, as the above-mentioned lithium transitionmetal composite oxide, a lithium transition metal composite oxide,having a crystal structure of an α-NaFeO₂ type, represented by acompositional formula Li_(1+α)Me_(1−α)O₂ (Me is a transition metalelement including Co, Ni and Mn, α>0) and having a molar ratio Li/Me ofLi to the transition metal element Me of 1.25 to 1.40, is used, andwherein charge in the above-mentioned initial charge-discharge isperformed up to any potential within the range of 4.5 V (vs. Li/Li⁺) ormore and less than 4.6 V (vs. Li/Li⁺) as a maximum achieved potential ofa positive electrode without generating oxygen gas from the lithiumtransition metal composite oxide.

(21) The method of manufacturing a nonaqueous electrolyte secondarybattery according to the above (20), wherein the positive activematerial has a plateau potential in a charge range, and any potentialwithin the range of 4.5 V (vs. Li/Li⁺) or more and less than 4.6 V (vs.Li/Li⁺) as a maximum achieved potential of a positive electrode is theplateau potential or more.

Advantages of the Invention

(a) In accordance with the above-mentioned means (1) to (13) of thepresent invention, a positive active material for a nonaqueouselectrolyte secondary battery having a large discharge capacity andexcellent charge-discharge cycle performance can be provided.

(b) In accordance with the above-mentioned means (2) and (4) to (6) ofthe present invention, a positive active material for a nonaqueouselectrolyte secondary battery having excellent initial efficiency inaddition to the effect of the above (a) can be provided.

(c) In accordance with the above-mentioned means (3) of the presentinvention, a positive active material for a nonaqueous electrolytesecondary battery having excellent high rate discharge performance inaddition to the effect of the above (a) can be provided.

(d) In accordance with the above-mentioned means (5) and (6) of thepresent invention, a positive active material for a nonaqueouselectrolyte secondary battery having excellent initial efficiency andexcellent high rate discharge performance in addition to the effect ofthe above (a) can be provided.

(e) In accordance with the above-mentioned means (8) to (12) of thepresent invention, a positive active material for a nonaqueouselectrolyte secondary battery not generating oxygen gas even when abattery is charged at a high voltage in addition to the effect of theabove (a) can be provided.

(f) In accordance with the above-mentioned means (14) to (16) of thepresent invention, a method of manufacturing a positive active materialfor a nonaqueous electrolyte secondary battery which exerts the effectsof the above (a) to (e) can be provided.

(g) In accordance with the above-mentioned means (17) to (19) of thepresent invention, it is possible to provide an electrode containing apositive active material for a nonaqueous electrolyte secondary batterywhich exerts the effects of the above (a) to (e), and a batteryincluding the electrode.

(h) In accordance with the above-mentioned means (20) and (21) of thepresent invention, it is possible to provide a method of manufacturing anonaqueous electrolyte secondary battery which uses a positive activematerial not generating oxygen gas even when a battery is charged at ahigh voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction chart at each electrochemical oxidationstage of a positive active material according to Example 1-1.

FIG. 2 is an X-ray diffraction chart at each electrochemical oxidationstage of a positive active material according to Comparative Example1-1.

FIG. 3A is an X-ray diffraction chart at each electrochemical oxidationstage of a positive active material according to Comparative Example 1-2and FIG. 3B is a partial enlarged view of X-ray diffraction chart ofFIG. 3A.

FIG. 4A is an X-ray diffraction chart at each electrochemical oxidationstage of a positive active material partial enlarged view ComparativeExample 1-3 and FIG. 4B is a partial enlarged view of X-ray diffractionchart of FIG. 4A.

FIG. 5 is an X-ray diffraction chart in electrochemically oxidizing thepositive active materials of Examples 2-1, 2-2 and Comparative Examples2-1 to 2-5 to a potential of 5.0 V (vs. Li/Li⁺).

FIG. 6 is a reference view for explaining an oxygen position parameter.

FIG. 7 is a view showing an embodiment of the present invention, whichis a longitudinal sectional view of a prismatic lithium secondarybattery.

FIG. 8 is a view showing potential behavior during an initialcharge-discharge step of a battery 1 in Example 3.

FIG. 9 is a view showing potential behavior during an initialcharge-discharge step of a battery 2 in Example 3.

FIG. 10 is a view showing potential behavior during an initialcharge-discharge step of a battery 3 in Example 3.

FIG. 11 is a view showing potential behavior during an initialcharge-discharge step of a battery 4 in Example 3.

MODE FOR CARRYING OUT THE INVENTION

With respect to the composition of the lithium transition metalcomposite oxide contained in the active material for a nonaqueouselectrolyte secondary battery of the present invention, the lithiumtransition metal composite oxide may contain the transition metalelements including Co, Ni and Mn, and Li, a molar ratio Li/Me of Li tothe transition metal element Me may be 1.2 to 1.6, a molar ratio Co/Meof Co in the transition metal element Me may be 0.02 to 0.23, and amolar ratio Mn/Me of Mn in the transition metal element Me may be 0.62to 0.72 in that a high discharge capacity is attained.

The lithium transition metal composite oxide is represented by thegeneral formula Li_(a)Co_(x)Ni_(y)Mn_(z)O₂ (a+x+y+z=2), and it ispreferred that a/(x+y+z) is 1.2 to 1.6, x/(x+y+z) is 0.02 to 0.23, andz/(x+y+z) is 0.62 to 0.72.

A lithium secondary battery having a large discharge capacity can beattained when the lithium transition metal composite oxide, whichsatisfies the requirements that a molar ratio Li/Me of Li to thetransition metal element Me is 1.2 to 1.6 (a/(x+y+z) is 1.2 to 1.6), amolar ratio Co/Me of Co to the Me is 0.02 to 0.23 (x/(x+y+z) is 0.02 to0.23), and a molar ratio Mn/Me of Mn to the Me is 0.62 to 0.72(z/(x+y+z) is 0.62 to 0.72), is used as an active material.

Since when a molar ratio Li/Me of Li to the transition metal element Meis less than 1.2, or the Li/Me is more than 1.6, a discharge capacity issmall, the Li/Me is set to the range of 1.2 to 1.6 (a/(x+y+z) is 1.2 to1.6) in order to attain a lithium secondary battery having a largedischarge capacity.

It is preferred to select a lithium transition metal composite oxide inwhich the molar ratio Li/Me of Li to the transition metal element Me is1.25 to 1.40 particularly from the viewpoint of being able to obtain anonaqueous electrolyte secondary battery having high initial efficiency.Moreover, it is preferred to set the molar ratio Li/Me of Li to thetransition metal element Me to the range of 1.250 to 1.350 in order toimprove the initial efficiency and the high rate discharge performance.

Since when a molar ratio Co/Me of Co to the transition metal element Meis less than 0.02, or the Co/Me is more than 0.23, a discharge capacityis small and initial efficiency is low, the Co/Me is set to the range of0.02 to 0.23 (x/(x+y+z) is 0.02 to 0.23) in order to attain a lithiumsecondary battery having a large discharge capacity and high initialefficiency. Moreover, the Co/Me is preferably 0.040 to 0.195 in order toimprove the high rate discharge performance.

Since when a molar ratio Mn/Me of Mn to the transition metal element Meis less than 0.62, a discharge capacity is small, and when the Mn/Me ismore than 0.72, a discharge capacity is small and initial efficiency islow, the Mn/Me is set to the range of 0.62 to 0.72 (z/(x+y+z) is 0.62 to0.72) in order to attain a lithium secondary battery having a largedischarge capacity and high initial efficiency. Moreover, the Mn/Me ispreferably 0.625 to 0.707 in order to improve the high rate dischargeperformance.

The lithium transition metal composite oxide of the present invention isrepresented by the general formula described above, and is essentially acomposite oxide composed of Li, Co, Ni and Mn, but it is not excludedthat the lithium transition metal composite oxide contains a smallamount of other metals such as alkali metals, for example, Na and Ca,alkaline earth metals, or transition metals typified by 3d transitionmetals such as Fe and Zn to an extent not impairing the effect of thepresent invention.

The lithium transition metal composite oxide of the present inventionhas an α-NaFeO₂ structure. The lithium transition metal composite oxideof the present invention can belong to P3₁12 or R3-m as a space group.Here, P3₁12 is a crystal structure model in which atom positions at 3a,3b and 6c sites in R3-m are subdivided, and the P3₁12 model is employedwhen there is orderliness in atom arrangement in R3-m. In addition,“R3-m” should be essentially written with a bar “-” added above “3” of“R3m”.

In the present specification, the crystal structure will be described byuse of Miller index serving as such as “(003) line”, “(104) line”, “(108line)” and “(110) line”, and these are Miller index in the case ofattributed to a space group R3-m as a crystal structure model. On theother hand, in the case of attributed to a space group P3₁12 as acrystal structure model, the Miller indices corresponding to the aboveexpression are respectively “(003) line”, “(114) line”, “(118) line” and“(300) line”. Accordingly, in the case of attributed to a space groupP3₁12 as a crystal structure model, the expression of “(104) line” inthe present specification is read as “(114) line”, the expression of“(108) line” is read as “(118) line”, and the expression of “(110) line”is read as “(300) line”.

The lithium transition metal composite oxide of the present inventionis, as described above, characterized by being observed as a singlephase attributed to a space group R3-m on an X-ray diffraction chartwhen the lithium transition metal composite oxide is electrochemicallyoxidized up to a potential of 5.0 V (vs. Li/Li⁺). Thereby, a nonaqueouselectrolyte secondary battery having excellent charge-discharge cycleperformance can be obtained, as shown in Examples described later.

Here, “when the lithium transition metal composite oxide iselectrochemically oxidized up to a potential of 5.0 V (vs. Li/Li⁺)” maybe when an electrochemical cell provided with an electrode containing anactive material for a nonaqueous electrolyte secondary batterycontaining a lithium transition metal composite oxide as a workingelectrode, and a counter electrode, a reference electrode and anelectrolyte is configured, and a potential of the working electroderelative to a potential of the reference electrode composed of lithiummetal is set to 5.0 V, and the specific condition may be as is describedin Examples described later.

Further, in order to satisfy the requirement of “being observed as asingle phase attributed to a space group R3-m on an X-ray diffractionchart”, it is enough that a diffraction pattern obtained by X-raydiffraction measurement attributes to a space group R3-m and that splitis not visually observed in a peak attributed to Miller index (003) linewhen a peak exhibiting the maximum intensity in the diffraction patternis drawn so as to fall within a full scale of a diffraction chart. Acommon apparatus like an X-ray diffraction measurement apparatus using aCuKα radiation source can be used for this measurement. When the CuKαradiation source is used, the peak attributed to the Miller index (003)line is observed around 19°. Although in measurement in ComparativeExample 1-1 described later, SPring-8 was employed, such a measurementmethod or measurement condition need not be necessarily employed, and byemploying a common apparatus like an X-ray diffraction measurementapparatus using a CuKα radiation source as employed in other Examples orComparative Examples, it is possible to distinguish whether “the lithiumtransition metal composite oxide is observed as a single phaseattributed to a space group R3-m on an X-ray diffraction chart”, or not.

In addition, the lithium transition metal composite oxide of the presentinvention can also be defined as “being observed as a single phase of ahexagonal crystal structure on an X-ray diffraction chart”, but sinceX-ray diffraction is performed in a charge state, if the lithiumtransition metal composite oxide is a single phase of a hexagonalcrystal structure, it attributes to a space group R3-m. The single phaseof the hexagonal crystal structure attributes to a space group P3₁12immediately after synthesis.

Moreover, in the present invention, it is required that an oxygenposition parameter, determined by crystal structure analysis by aRietveld method based on an X-ray diffraction pattern, is 0.260 or less.When the oxygen position parameter is 0.260 or less, a nonaqueouselectrolyte secondary battery having excellent high rate dischargeperformance can be attained. In addition, as shown in Examples describedlater, the positive active material in a state of a discharge end isused for a sample to be subjected to X-ray diffraction measurement forobtaining an X-ray diffraction pattern which forms a foundation fordetermining an oxygen position parameter. Accordingly, when an oxygenposition parameter is evaluated on the active material for a nonaqueouselectrolyte secondary battery contained in a positive electrode obtainedby disassembling a nonaqueous electrolyte battery, it is necessary topreviously bring the nonaqueous electrolyte secondary battery into astate of a discharge end by low rate discharge before disassembling thebattery. Further, since a material of an active material aftersynthesis, that is, a material of an active material before used for anelectrode for a nonaqueous electrolyte secondary battery, can be said tobe in a state of a discharge end, the active material may be directlysubjected to X-ray diffraction measurement as-is when an oxygen positionparameter is evaluated on the active material.

In the present specification, the oxygen position parameter refers to avalue of z at the time when with respect to a crystal structure of anα-NaFeO₂ type of a lithium transition metal composite oxide attributedto a space group R3-m, a space coordinate of Me (transition metal) isdefined as (0, 0, 0), a space coordinate of Li (lithium) is defined as(0, 0, ½), and a space coordinate of O (oxygen) is defined as (0, 0, z).That is, the oxygen position parameter is a relative mark indicating howfar an O (oxygen) position is from a Me (transition metal) position.FIG. 6 is shown as a reference drawing.

In the present invention, a BET specific surface area is preferably 0.88m²/g or more, and more preferably 1.24 to 5.87 m²/g in order to obtain alithium secondary battery having excellent initial efficiency andexcellent high rate discharge performance.

A tapped density is preferably 1.25 g/cm³ or more, and more preferably1.44 g/cm³ or more particularly in order to obtain a lithium secondarybattery having excellent high rate discharge performance.

The positive active material of the present invention is characterizedin that oxygen gas is not generated from the above lithium transitionmetal composite oxide when charge is performed up to any potentialwithin the range of 4.5 to 4.6 V (vs. Li/Li⁺) as a maximum achievedpotential of a positive electrode. Since in a conventional positiveactive materials, as shown in Patent Documents 11, 12, and Non-patentDocuments 3, 4, when the lithium transition metal composite oxide is alithium-excess transition metal composite oxide, a maximum achievedpotential of a positive electrode is about 4.5 V (vs. Li/Li⁺) and oxygengas is generated from the composite oxide at this potential, it is saidthat the positive active material of the present invention is completelydifferent from the conventional positive active materials. In thepresent invention, it has been confirmed that oxygen gas is notgenerated from the lithium transition metal composite oxide when chargeis performed up to each potential of 4.5 V, 4.55 V, and 4.6 V (vs.Li/Li⁺) as a maximum achieved potential of a positive electrode, asshown in Examples described later.

In the present invention, “oxygen gas is not generated” means thatoxygen gas is not substantially generated, and specifically, a volumeratio (O₂/(N₂+O₂) of oxygen to the total amount of nitrogen and oxygenis preferably 0.20 to 0.50, and more preferably 0.20 to 0.25 in the casewhere a sealed battery containing a positive active material with thecomposition of the present invention is prepared, and charge isperformed up to any potential within the range of 4.5 to 4.6 V (vs.Li/Li⁺) and discharge is performed up to 2.0 V (vs. Li/Li⁺), and thenthe battery is disassembled and gas released from the battery inside isanalyzed by using gas chromatography. It is particularly preferred thata volume ratio between oxygen and nitrogen is not different (measurementerror: ±5%) from an atmospheric component (O₂/(N₂+O₂)=0.21), that is, anamount of generated oxygen gas is below the detection limit. Inaddition, charge-discharge of the battery is performed at ordinarytemperature (25° C.) and is not performed in an extremely heated state.

When as described above, the maximum achieved potential of a positiveelectrode is within the range of 4.5 to 4.6 V (vs. Li/Li⁺), oxygen gasis not generated even if charge is performed up to any potential, butwhen the charge is performed as the charge in the initial formation(initial charge-discharge) in a manufacturing step of a lithiumsecondary battery, charge is preferably performed up to any potentialwithin the range of more than 4.5 V (vs. Li/Li⁺) and less than 4.6 V(vs. Li/Li⁺) as the maximum achieved potential of a positive electrode,and more preferably performed up to 4.55 V or a potential close to 4.55V.

When the maximum achieved potential of a positive electrode is 4.5 V(vs. Li/Li⁺) or less, a discharge capacity is small in a dischargeregion of 4.3 V (vs. Li/Li⁺) or less, and when the maximum achievedpotential of a positive electrode is 4.6 V (vs. Li/Li⁺) or more, anamount of gas generated through decomposition of an electrolyte solutionis increased though the discharge capacity is increased, resulting inthe deterioration of battery performance, and therefore these ranges arenot preferred.

When charge up to 4.55 V (vs. Li/Li⁺) as the maximum achieved potentialof a positive electrode is performed as the charge in the initialcharge-discharge (first charge-discharge) as in Example described later,a lithium secondary battery can be manufactured without generatingoxygen gas, and the lithium secondary battery thus manufactured canattain a large discharge capacity when a charge method, in which themaximum achieved potential of a positive electrode at the time of chargeis 4.3 V (vs. Li/Li⁺), is employed at the time of use.

Further, the positive active material of the present invention has aplateau potential in a charge range, and any potential within the rangeof 4.5 to 4.6 V (vs. Li/Li⁺) is the plateau potential or more. Here, theplateau potential means “a region where a potential change appearing tothe amount of charge in the range of a positive electrode potential isrelatively flat” as shown in FIG. 9 of Patent Document 1, and has thesame meaning as that described in Patent Document 11.

The present invention is characterized in that oxygen gas is notsubstantially generated from a lithium transition metal composite oxideeven when charge is performed at a plateau potential or more in initialcharge-discharge (first charge-discharge) of a lithium secondary batteryusing the positive active material which contains a lithium transitionmetal composite oxide having a molar ratio Li/Me of Li to all transitionmetal elements Me (Co, Ni and Mn) of 1.25 to 1.40, and has the plateaupotential in the charge range.

Next, a method of manufacturing an active material for a nonaqueouselectrolyte secondary battery of the present invention will bedescribed.

The active material for a nonaqueous electrolyte secondary battery ofthe present invention can be obtained basically by adjusting a rawmaterial so as to contain metal elements (Li, Mn, Co, Ni) composing theactive material just as the intended composition of the active material(lithium transition metal composite oxide), and finally sintering theraw material. However, an amount of a Li material loaded is preferablyexcessive by about 1 to 5% considering that a part of the Li material isdisappeared during sinter.

As a method for preparing a lithium transition metal composite oxidehaving the intended composition, the so-called “solid state method” inwhich salts of Li, Co, Ni, and Mn are mixed and sintered, and“coprecipitation method” in which a coprecipitated precursor in whichCo, Ni, and Mn exist in one particle is previously prepared, and theprecursor is mixed with a Li salt and the resulting mixture is sinteredare known. In a synthesis process by the “solid state method”,particularly Mn is hard to be homogeneously solid soluted in Co or Ni.Therefore, it is difficult to obtain a sample in which the respectiveelements are distributed homogeneously in one particle. In producing theactive material for a nonaqueous electrolyte secondary battery of thepresent invention, selection of the “solid state method” and“coprecipitation method” is not particularly limited. However, when the“solid state method” is selected, it is extremely difficult to producethe positive active material of the present invention. Selection of the“coprecipitation method” is preferred in that an active material inwhich each element distribution is more homogeneous can be easilyobtained.

In preparing the coprecipitated precursor, since Mn among Co, Ni and Mnis easily oxidized, and it is not easy to prepare the coprecipitatedprecursor in which Co, Ni and Mn are homogeneously distributed in adivalent state, uniform mixing of Co, Ni and Mn at an element leveltends to be insufficient. Particularly, in the range of the compositionof the present invention, a ratio of Mn is larger than those of Co andNi, it is important to remove dissolved oxygen in an aqueous solution. Amethod of removing dissolved oxygen includes a method of bubbling a gasnot containing oxygen. The gas not containing oxygen is not particularlylimited, and nitrogen gas, argon gas, carbon dioxide (CO₂) can be used.Particularly in the case where a coprecipitated carbonate precursor isprepared as in Example described later, if carbon dioxide is employed asthe gas not containing oxygen, it is preferred since an environment inwhich carbonate is easily produced is provided.

A pH in the step of coprecipitating a compound containing Co, Ni and Mnin a solution to produce a precursor is not limited, and the pH can be8.5 to 11 when the coprecipitated precursor is prepared as acoprecipitated carbonate precursor. In order to increase a tappeddensity, it is preferred to control a pH. When the pH is adjusted to 9.4or less, the tapped density can be 1.25 g/cm³ or more to improve thehigh rate discharge performance.

For preparation of the coprecipitation hydroxide precursor, a compoundis preferable in which Mn, Ni and Co are homogeneously mixed. However,the precursor is not limited to a hydroxide and besides, an insolublesalt in which elements homogeneously exist at an element level, such asa carbonate or a citrate, can be used similarly to a hydroxide. Aprecursor having a higher bulk density can also be prepared by using acrystallization reaction or the like using a complexing agent. At thistime, by mixing and calcinating with a Li source and a Na source, anactive material having a high density and a small specific surface areacan be obtained, and therefore the energy density per electrode area canbe improved.

Examples of the raw material of the coprecipitation hydroxide precursormay include manganese oxide, manganese carbonate, manganese sulfate,manganese nitrate and manganese acetate as a Mn compound, nickelhydroxide, nickel carbonate, nickel sulfate and nickel acetate as a Nicompound, and cobalt sulfate, cobalt nitrate and cobalt acetate as a Cocompound.

As a raw material for preparation of the coprecipitation hydroxideprecursor, a material in any form can be used as long as it forms aprecipitation reaction with an aqueous alkali solution, but it ispreferable that a metal salt having a high solubility be used.

The active material for a lithium secondary battery in the presentinvention can be suitably prepared by mixing the coprecipitationhydroxide precursor with a Li compound, followed by heat-treating themixture. The active material can be suitably produced by using lithiumhydroxide, lithium carbonate, lithium nitrate, lithium acetate or thelike as a Li compound.

For obtaining an active material, which has a high reversible capacity,selection of the sintering temperature is very important.

If the sintering temperature is too high, the obtained active materialis collapsed with an oxygen release reaction, a phase defined as aLi[Li_(1/3)Mn_(2/3)]O₂ type of a monoclinic crystal, in addition to ahexagonal crystal as a main phase tends to be observed as a separatephase rather than a solid solution phase, and such a material is notpreferable because the reversible capacity of the active materialsignificantly decreases. Therefore, it is important to ensure that thesintering temperature is lower than a temperature at which the oxygenrelease reaction of the active material is influential. The oxygenrelease temperature of the active material is generally 1000° C. orhigher in the composition range according to the present invention, butsince the oxygen release temperature slightly varies depending on thecomposition of the active material, it is preferable to check the oxygenrelease temperature of the active material beforehand. Particularly, itshould be noted that the oxygen release temperature has been found toshift toward the low temperature side as the amount of Co contained inthe active material increases. As a method for checking the oxygenrelease temperature of the active material, a mixture of acoprecipitation precursor with LiOH.H₂O may be subjected tothermogravimetric analysis (DTA-TG measurement) for simulating a sinterreaction process, but in this method, platinum used in a sample chamberof a measuring instrument may be corroded by a volatilized Li componentto damage the instrument, and therefore a composition that iscrystallized on some level beforehand by employing a sinteringtemperature of about 500° C. should be subjected to thermogravimetricanalysis.

On the other hand, if the sintering temperature is too low,crystallization does not sufficiently proceed, and active materialcharacteristics are significantly degraded, thus being not preferable.The sintering temperature is required to be at least 800° C. Sufficientcrystallization is important for reducing the resistance of a crystalgrain boundary to facilitate smooth transportation of lithium ions.Examples of the method for determining the degree of crystallizationinclude visual observation using an scanning electron microscope. Thepositive active material of the present invention was observed with ascanning electron microscope to find that the positive active materialwas formed of nano-order primary particles at the active materialsynthesis temperature of 800° C. or lower, but was crystallized to asub-micron level by further elevating the active material synthesistemperature, and large primary particles leading to improvement ofactive material characteristics could be obtained.

The inventors precisely analyzed the half width of the active materialof the present invention to find that a strain remained in a lattice inthe active material synthesized at a temperature of up to 800° C., andthe strain could be mostly removed by synthesizing the active materialat a higher temperature. The size of the crystallite was increasedproportionally as the synthesis temperature was elevated. Therefore, inthe composition of the active material of the present invention, a gooddischarge capacity was also obtained by aiming for particles in whichthe strain of the lattice is little present in a lattice, and thecrystallite size is sufficiently grown. Specifically, it has been foundthat it is preferable to employ such a synthesis temperature (sinteringtemperature) that the amount of strain having an effect on the latticeconstant is 1% or less, and the crystallite size is grown to 100 nm ormore. When these particles are molded as an electrode andcharge-discharge is performed, a change occurs due to expansion andcontraction, but it is preferable for effect of present invention thatthe crystallite size to be kept at 50 nm or more even in acharge-discharge process. That is, an active material having anexcellent initial efficiency and a high reversible capacity can beobtained only by selecting the calcination temperature so as to be asclose as possible to the above-described oxygen release temperature ofthe active material.

As described above, while a preferred sintering temperature variesdepending on an oxygen-release temperature of the active material andtherefore it is difficult to set a preferred range of the sinteringtemperature comprehensively, in the present invention, and it ispreferred to set the sintering temperature to the range of 800 to 1000°C. in order to make the discharge capacity sufficient when the molarratio Li/Me is 1.2 to 1.6. Speaking more, the sintering temperature ispreferably around 800 to 940° C. when the molar ratio Li/Me is below1.5, and is preferably around 1000° C. when the molar ratio Li/Me is 1.5to 1.6.

The nonaqueous electrolyte used in the nonaqueous electrolyte secondarybattery according to the present invention is not limited, and thosethat are generally proposed to be used in lithium batteries and the likecan be used. Examples of the nonaqueous solvent used in the nonaqueouselectrolyte may include, but are not limited to, cyclic carbonates suchas propylene carbonate, ethylene carbonate, butylene carbonate,chloroethylene carbonate and vinylene carbonate; cyclic esters such asγ-butyrolactone and γ-valerolactone; chain carbonates such as dimethylcarbonate, diethyl carbonate and ethylmethyl carbonate; chain esterssuch as methyl formate, methyl acetate and methyl butyrate;tetrahydrofuran or derivatives thereof; ethers such as 1,3-dioxane,1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane and methyl diglyme;nitriles such as acetonitrile and benzonitrile; dioxolane or derivativesthereof; and ethylene sulfide, sulfolane, sultone or derivatives thereofalone or mixtures of two or more thereof.

Examples of the electrolyte salt used in the nonaqueous electrolyteinclude inorganic ion salts including one of lithium (Li), sodium (Na)and potassium (K), such as LiClO₄, LiBF₄, LiAsF₆, LiPF₆, LiSCN, LiBr,LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, NaClO₄, NaI, NaSCN, NaBr, KClO₄ and KSCN, andorganic ion salts such as LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂,LiN(CF₃SO₂)(C₄F₉SO₂), LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, (CH₃)₄NBF₄,(CH₃)₄NBr, (C₂H₅)₄NClO₄, (C₂H₅)4NI, (C₃H₇)₄NBr, (n-C₄H₉)₄NClO₄,(n-C₄H₉)₄NI, (C2H₅)₄N-maleate, (C₂H₅)₄N-benzoate, (C₂H₅)₄N-phtalate,lithium stearylsulfonate, lithium octylsulfonate and lithiumdodecylbenzenesulfonate, and these ionic compounds can be used alone orin combination of two or more thereof.

Further, by mixing LiPF₆ or LiBF₄ with a lithium salt having aperfluoroalkyl group, such as LiN (C₂F₅SO₂)₂, the viscosity of theelectrolyte can be further reduced, so that the low-temperaturecharacteristics can be further improved, and self discharge can besuppressed, thus being more desirable.

A salt that is melted at ordinary temperature or an ion liquid may beused as a nonaqueous electrolyte.

The concentration of the electrolyte salt in the nonaqueous electrolyteis preferably 0.1 mol/l to 5 mol/l, further preferably 0.5 mol/l to 2.5mol/l for reliably obtaining a nonaqueous electrolyte battery havinghigh battery characteristics.

The negative electrode material is not limited, and may be freelyselected as long as it can precipitate or absorb lithium ions. Examplesthereof include titanium-based materials such as lithium titanate havinga spinel-type crystal structure represented by Li[Li_(1/3)Ti_(5/3)]O₄,alloy-based materials such as Si-, Sb- and Sn-based alloy materials,lithium metals, lithium alloys (lithium metal-containing alloys such aslithium-silicon, lithium-aluminum, lithium-lead, lithium-tin,lithium-aluminum-tin, lithium-gallium and wood alloys), lithiumcomposite oxides (lithium-titanium) and silicon oxide as well as alloyscapable of absorbing/releasing lithium, and carbon materials (e.g.,graphite, hard carbon, low temperature-fired carbon and amorphouscarbon).

It is desirable that the powder of the positive active material and thepowder of the negative electrode material have an average particle sizeof 100 μm or less. Particularly, it is desirable that the powder of thepositive active material have a size of 10 μm or less for the purpose ofimproving the high power characteristics of the nonaqueous electrolytebattery. A crusher and a classifier are used for obtaining a powder in apredetermined shape. For example, a mortar, a ball mill, a sand mill, avibration ball mill, a planet ball mill, a jet mill, a counter jet mill,a revolving airflow-type jet mill, a sieve or the like is used. At thetime of crushing, wet crushing can also be used in which water, or anorganic solvent such as hexane coexists. The classification method isnot particularly limited, and a sieve, an air classifier or the like isused as necessary in both dry and wet processes.

The positive active material and the negative electrode material whichare main components of the positive electrode and the negative electrodehave been described in detail above, but the positive electrode andnegative electrode may contain, in addition to the main components, aconducting agent, a binder, a thickener, a filler and the like as othercomponents.

The conducting agent is not limited as long as it is anelectron-conductive material that has no adverse effect on batteryperformance, but normally conductive materials such as natural graphite(scaly graphite, flake graphite, earthy graphite, etc.), artificialgraphite, carbon black, acetylene black, ketjen black, carbon whisker,carbon fibers, metal (copper, nickel, aluminum, silver, gold, etc.)powders, metal fibers and conductive ceramic materials can be includedalone or as a mixture thereof.

Among them, acetylene black is desirable as a conducting agent from theviewpoints of electron conductivity and coating properties. The addedamount of the conducting agent is preferably 0.1% by weight to 50% byweight, especially preferably 0.5% by weight to 30% by weight based onthe total weight of the positive electrode or negative electrode.Particularly, use of acetylene black crushed into ultrafine particles of0.1 to 0.5 μm is desirable because the required amount of carbon can bereduced. These mixing methods involve physical mixing, the ideal ofwhich is homogeneous mixing. Thus, mixing can be carried out in a dryprocess or a wet process using a powder mixer such as a V-type mixer, anS-type mixer, a grinder, a ball mill or a planet ball mill.

As the binder, thermoplastic resins such as polytetrafluoroethylene(PTFE), polyvinylidene fluoride (PVDF), polyethylene and polypropylene,and polymers having rubber elasticity, such as ethylene-propylene-dieneterpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR) andfluororubber can normally be used alone or as a mixture of two or morethereof. The added amount of the binder is preferably 1 to 50% byweight, especially preferably 2 to 30% by weight based on the totalweight of the positive electrode or negative electrode.

The filler may be any material as long as it has no adverse effect onbattery performance. An olefin-based polymer such as polypropylene orpolyethylene, amorphous silica, alumina, zeolite, glass, carbon or thelike is normally used. The added amount of the filler is preferably 30%by weight or less based on the total amount of the positive electrode orthe negative electrode.

The positive electrode and the negative electrode are suitably preparedby kneading the aforementioned main components (positive active materialin the positive electrode and negative electrode material in thenegative electrode) and other materials to form a mixture, and mixingthe mixture with an organic solvent, such as N-methylpyrrolidone ortoluene, or water, followed by applying or contact-bonding the resultingmixed liquid onto a current collector that is described in detail below,and carrying out a heating treatment at a temperature of about 50° C. to250° C. for about 2 hours. For the applying method, for example, it isdesirable to perform applying in any thickness and any shape using meanssuch as roller coating by an applicator roll or the like, screencoating, a doctor blade system, spin coating or a bar coater, but theapplying method is not limited thereto.

As a separator, it is preferable that a porous membrane, a nonwovenfabric or the like, which shows excellent high rate dischargeperformance, be used alone or in combination. Examples of the materialthat forms the separator for a nonaqueous electrolyte battery mayinclude polyolefin-based resins represented by polyethylene,polypropylene and the like, polyester-based resins represented bypolyethylene terephthalate, polybutylene terephthalate and the like,polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylenecopolymers, vinylidene fluoride-perfluoro vinyl ether copolymers,vinylidene fluoride-tetrafluoroethylene copolymers, vinylidenefluoride-trifluoroethylene copolymers, vinylidenefluoride-fluoroethylene copolymers, vinylidenefluoride-hexafluoroacetone copolymers, vinylidene fluoride-ethylenecopolymers, vinylidene fluoride-propylene copolymers, vinylidenefluoride-trifluoropropylene copolymers, vinylidenefluoride-tetrafluoroethylene-hexafluoropropylene copolymers andvinylidene fluoride-ethylene-tetrafluoroethylene copolymers.

The porosity of the separator is preferably 98% by volume or less fromthe viewpoint of the strength. The porosity is preferably 20% by volumeor more from the viewpoint of charge-discharge characteristics.

For the separator, for example, a polymer gel formed of acrylonitrile,ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate,vinyl pyrrolidone or a polymer such as poly(fluoride vinylidene) and anelectrolyte may be used. Use of the nonaqueous electrolyte in a gel formas described above is preferable from the viewpoint of being effectiveto prevent liquid leakage.

Further, for the separator, use of the porous membrane, nonwoven fabricor the like in combination with the polymer gel is desirable becauseliquid retainability of the electrolyte is improved. That is, a filmwith the surface and the microporous wall face of a polyethylenemicroporous membrane coated with a solvophilic polymer in a thickness ofseveral μm or less is formed, and an electrolyte is held withinmicropores of the film, so that the solvophilic polymer is formed into agel.

Examples of the solvophilic polymer include, in addition topoly(fluoride vinylidene), polymers in which an acrylate monomer havingan ethylene oxide group, an ester group or the like, an epoxy monomer, amonomer having an isocyanate group, or the like is crosslinked. Themonomer can be subjected to a crosslinking reaction by carrying outheating or using ultraviolet rays (UV) while using a radical initiatorat the same time, or using active light rays such as electron beams(EB), or the like.

The configuration of the nonaqueous electrolyte secondary battery is notparticularly limited, and examples thereof include a cylindrical batteryhaving a positive electrode, a negative electrode and a roll-shapedseparator, a prismatic battery and a flat battery.

Both the conventional positive active material and the active materialof the present invention are capable of charge-discharge at a positiveelectrode potential of around 4.5 V (vs. Li/Li⁺). However, depending onthe type of using nonaqueous electrolyte, the nonaqueous electrolyte maybe oxidatively decomposed to cause deterioration of battery performancebecause the positive electrode potential during charge is too high. Anonaqueous electrolyte secondary battery, with which a sufficientdischarge capacity is obtained even when such a charge method that themaximum potential of the positive electrode during charge is 4.3 V (vs.Li/Li⁺) or less is employed at the time of operation, may be required.If the active material of the present invention is used, a dischargeelectrical amount, which exceeds the capacity of the conventionalpositive active material, i.e., about 200 mAh/g, can be obtained evenwhen such a charge method that the maximum potential of the positiveelectrode during charge is lower than 4.5 V (vs. Li/Li⁺), for example,4.4 (vs. Li/Li⁺) or less or 4.3 (vs. Li/Li⁺) or less is employed at thetime of user operation.

For the positive active material according to the present invention tohave a high discharge capacity, the ratio, at which transition metalelements that form a lithium-transition metal composite oxide arepresent in areas other than transition metal sites of a layeredrock-salt-type crystal structure, is preferably low. This can beachieved by ensuring that in precursor particles that are subjected to asintering step, transition metal elements such as Co, Ni and Mn inprecursor are sufficiently homogeneously distributed, and selectingsuitable conditions for the sintering step for promoting crystallizationof an active material. If distribution of transition metals in precursorcore particles that are subjected to the sintering step is nothomogeneous, a sufficient discharge capacity is not obtained. The reasonfor this is not necessarily clear, but the present inventors infer thatthis results from occurrence of so called cation mixing in which theobtained lithium-transition metal composite oxide has some of transitionmetal elements present in areas other than transition metal sites of thelayered rock-salt-type crystal structure, i.e., lithium sites if thedistribution of transition metals in precursor core particles that aresubjected to the sintering step is not homogeneous. A similar inferencecan be applied in a crystallization process in the sintering step,wherein cation mixing in the layered rock-salt-type crystal structureeasily occurs if crystallization of the active material is insufficient.Those in which the homogeneity of the distribution of the transitionmetal elements is high tend to have a high intensity ratio ofdiffraction peaks of the (003) line and the (104) line when the resultof X-ray diffraction measurement is attributed to a space group R3-m. Inthe present invention, the intensity ratio of diffraction peaks of the(003) line and the (104) line (attributed to a space group P3₁12 andbeing (114) line before charge-discharge) from X-ray diffractionmeasurement is preferably I₍₀₀₃₎/I₍₁₀₄₎≥1.20. The intensity ratio ispreferably I₍₀₀₃₎/I₍₁₀₄₎>1 in a state of complete discharge aftercharge-discharge. If synthesis conditions and synthesis procedures forthe precursor are inappropriate, the peak intensity ratio is a smallervalue, which is often less than 1.

By employing the synthesis conditions and synthesis procedures describedin the specification of the present application, a positive activematerial having high performance as described above can be obtained.Particularly, there can be provided a positive active material for anonaqueous electrolyte secondary battery with which a high dischargecapacity can be obtained even when the charge upper limit potential ofpositive electrode is set to lower than 4.5, e.g., a charge upper limitpotential such as 4.4 V or 4.3 V is set.

Example 1 Example 1-1

4.6873 g of cobalt sulfate heptahydrate, 6.5743 g of nickel sulfatehexahydrate and 22.110 g of manganese sulfate pentahydrate were weighed,and dissolved in 200 ml of ion-exchanged water to prepare a 0.67 Maqueous sulfate solution of which the molar ratio of Co:Ni:Mn was12.5:18.75:68.75. On the other hand, 750 ml of ion-exchanged water waspoured into a 2 dm³ reaction tank, and CO₂ was dissolved in theion-exchanged water by bubbling CO₂ gas for 30 minutes in theion-exchanged water. A temperature of the reaction tank was set to 50°C. (±2° C.), and the aqueous sulfate solution was added dropwise at arate of 3 ml/min while stirring the content in the reaction tank at arotational speed of 700 rpm using a paddle blade equipped with astirring motor. Here, from the start of the dropwise addition until thecompletion thereof, an aqueous solution containing 0.67 M of sodiumcarbonate and 0.067 M of ammonia was appropriately added dropwise tocontrol the content in the reaction tank so as to always maintain a pHof 8.6 (±0.05). After the completion of the dropwise addition, thestirring was continued for further one hour. After the stirring wasstopped, the solution was left standing for 12 hours or more.

Next, using a suction filtration apparatus, particles of coprecipitatedcarbonate salt produced in the reaction tank were separated, sodium ionsadhering to the particles were cleaned and removed with ion-exchangewater, and the resulting particles were dried at 100° C. under ordinarypressure in an air atmosphere by using an electric furnace. Thereafter,particles were pulverized for several minutes by using an automaticmortar made of agate in order to level particle sizes. In this way, acoprecipitated carbonate precursor was prepared.

Lithium carbonate (0.9436 g) was added to 2.3040 g of the coprecipitatedcarbonate precursor, and the resulting mixture was adequately mixed byusing an automatic mortar made of agate to prepare a mixed powder inwhich the molar ratio of Li and (Co, Ni, Mn) was 130:100. The mixedpowder was molded at a pressure of 6 MPa by using a pellet moldingmachine to be formed into pellets with a diameter of 25 mm. The amountof the mixed powder subjected to pellet molding was determined so as tobe equivalent to 2 g of a mass of an assumed final product. One of thepellets was placed on an aluminum boat having an entire length of about100 mm, placed together with the boat in a box type electric furnace(model number: AMF 20), and sintered at 900° C. for 10 hours underordinary pressure in an air atmosphere. Inner dimensions of the box typeelectric furnace were 10 cm long, 20 cm wide and 30 cm deep, and heatingwires were disposed in a width direction at 20-centimeter intervals.After sintering, a heater was turned off, and the aluminum boat wasnaturally cooled as it was left standing in the furnace. Consequently, atemperature of the furnace was lowered to about 200° C. after 5 hours,but a subsequent temperature lowering rate was slightly mild. After aelapse of a whole day and night, it was confirmed that a temperature ofthe furnace was 100° C. or lower, and then the pellet was taken out andpulverized for several minutes by using an automatic mortar made ofagate in order to level particle sizes. In this way, a lithiumtransition metal composite oxide of Example 1-1 was prepared.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.13)Co_(0.11)Ni_(0.16)Mn_(0.60)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation, a hexagonalcrystal structure of an α-NaFeO₂ type was identified as a main phase.

Comparative Example 1-1

An aqueous sulfate solution, in which the respective elements Co, Ni,and Mn were dissolved in the molar ratio of 12.5:18.75:68.75, wasprepared. On the other hand, a temperature of a reaction tank pouredwith ion-exchange water was maintained at 70° C., and to theion-exchange water, an aqueous NaOH solution was added dropwise toadjust a pH to 10.3. Next, dissolved oxygen was removed by bubbling aninert gas. An outlet was fixed in this reaction tank so that a solutionwas ejected from the outlet if a liquid level in the reaction tankexceeded a certain height. Further, a stirring blade was disposed in thereaction tank, and a cylindrical convection plate was fixed in thereaction tank in order to cause convection in a vertical directionduring stirring the solution. The aqueous sulfate solution was addeddropwise at a feed rate of 11.5 ml/min while stirring the content in thereaction tank. A part of a solution containing a reaction product wasejected out of the reaction tank from the outlet during the aqueoussulfate solution was added dropwise, but the ejected solution wasdiscarded without being returned to the reaction tank before thedropwise addition of all of the sulfate aqueous solution was completed.While the dropwise addition was continued, the temperature of thereaction tank was maintained at 70° C., and an aqueous NaOH solution wasappropriately added dropwise while monitoring a pH so that the pH alwaysfell within the range of 10.3±0.1. After the completion of the dropwiseaddition, the stirring was stopped and the solution was left standingfor 12 hours or more. Next, the resulting coprecipitated product wasseparated by filtration and dried at 140° C. under ordinary pressure inan air atmosphere by using an oven. After drying, the coprecipitatedproduct was pulverized lightly to an extent of leveling particle sizes.Thereby, a dried powder was obtained.

Lithium hydroxide was added to the obtained dried powder so as to havethe molar ratio of Li and (Co+Ni+Mn) of 150:100, and ethanol was furtheradded, and the resulting mixture was wet-mixed. About 5 kg of theresulting mixture was transferred to a sagger, and placed with thesagger in a furnace and sintered at 1000° C. After a temperature of thefurnace was returned to ordinary temperature, a sintered product wastaken out and pulverized to an extent of leveling particle sizes byusing a mortar. In this way, a lithium transition metal composite oxideof Comparative Example 1-1 was prepared.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.2)Co_(0.1)Ni_(0.15)Mn_(0.55)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation, a hexagonalcrystal structure of an α-NaFeO₂ type was identified as a main phase.

Comparative Example 1-2

Lithium carbonate (Li₂CO₃), cobalt hydroxide (Co(OH)₂), nickel hydroxide(Ni(OH)₂) and manganese oxyhydroxide (MnOOH) were weighed so as to havethe ratio of 150:12.5:18.75:68.75 of the respective elements Li, Co, Ni,and Mn, and these raw materials were adequately mixed and pulverizedusing a mortar. Next, 2 g of the resulting mixture was sintered at 1000°C. for 12 hours in the air. In this way, a lithium transition metalcomposite oxide of Comparative Example 1-2 was obtained.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.2)Co_(0.1)Ni_(0.15)Mn_(0.55)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation, a hexagonalcrystal structure of an α-NaFeO₂ type was identified as a main phase.

Comparative Example 1-3

Lithium carbonate (Li₂CO₃), cobalt hydroxide (Co(OH)₂), nickel hydroxide(Ni(OH)₂) and manganese oxyhydroxide (MnOOH) were weighed so as to havethe ratio of 130:12.6:18.4:69.0 of the respective elements Li, Co, Ni,and Mn, and these raw materials were adequately mixed and pulverizedusing a mortar. Next, 2 g of the resulting mixture was sintered at 900°C. for 10 hours in the air. In this way, a lithium transition metalcomposite oxide of Comparative Example 1-3 was obtained.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.13)Co_(0.11)Ni_(0.16)Mn_(0.60)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation, a hexagonalcrystal structure of an α-NaFeO₂ type was identified as a main phase.

(Assembling and Evaluation of Nonaqueous Electrolyte Secondary Battery)

Each of the lithium transition metal composite oxides in Example 1-1 andComparative Examples 1-1 to 1-3 was used as a positive active materialfor a nonaqueous electrolyte secondary battery, and a nonaqueouselectrolyte secondary battery was assembled by the following procedure,and battery characteristics thereof were evaluated.

A positive active material, acetylene black (AB) and polyvinylidenefluoride (PVdF) were respectively mixed in a mass ratio of 85:8:7. Tothis, N-methylpyrrolidone as a dispersion medium was added, and theresulting mixture was kneaded/dispersed to prepare a applying paste. Inaddition, a mass ratio of the PVdF was shown on a solid mass basis sincea liquid in which a solid content was dissolved/dispersed was used. Theapplying paste was applied onto an aluminum foil current collectorhaving a thickness of 20 μm to prepare a positive electrode plate.

A lithium metal was used for a counter electrode (negative electrode) inorder to observe the behavior of the positive electrode alone. Thelithium metal was closely attached to a nickel foil current collector.However, it was prepared in such a manner that the capacity of thenonaqueous electrolyte secondary battery was controlled adequately bythe positive electrode.

As an electrolyte solution, a solution obtained by dissolving LiPF₆, soas to have a concentration of 1 mol/1, in a mixed solvent in which avolume ratio of EC/EMC/DMC was 6:7:7 was used. A microporous membranemade of polypropylene, in which an electrolyte-retainbility was improvedby surface modification using polyacrylate, was used as a separator. Anickel plate, to which a lithium metal foil was attached, was used as areference electrode. A metal-resin composite film made of polyethyleneterephthalate (15 μm)/aluminum foil (50 μm)/metal-adhesive polypropylenefilm (50 μm) was used for a outer case. The respective electrodes werehoused in the outer case in such a way that open ends of a positiveelectrode terminal, a negative electrode terminal and a referenceelectrode terminal were exposed to outside. And fusion margins with theinner surfaces of the aforementioned metal resin composite films facingeach other were airtightly sealed except a portion forming anelectrolyte solution filling hole.

On the nonaqueous electrolyte secondary batteries thus assembled, aninitial charge-discharge step of two cycles was performed at 25° C. Thevoltage control was all performed for a positive electrode potential.Charge was constant current-constant voltage charge with a current of0.1 CmA and a voltage of 4.6 V. The charge termination condition was setat a time point at which the current value decreased to 0.02 CmA.Discharge was constant current discharge under the conditions of acurrent of 0.1 CmA and a final voltage of 2.0 V. In all the cycles, arest time of 30 minutes was set after charge and after discharge.

Subsequently, a charge-discharge cycle test was performed. The voltagecontrol was all performed for a positive electrode potential. Theconditions of the charge-discharge cycle test were the same as theconditions of the above initial charge-discharge step except for settingthe charge voltage to 4.3 V (vs. Li/Li⁺). In all cycles, a rest time of30 minutes was set after charge and after discharge. The dischargeelectrical quantity at the 1st cycle and the discharge electricalquantity at the 30th cycle in the charge-discharge cycle test wererespectively recorded as “discharge capacity (mAh)”, and a ratio of thedischarge electrical quantity at the 30th cycle to the dischargeelectrical quantity at the 1st cycle was recorded as “capacity retentionratio (%)”. The results are shown in Table 1.

TABLE 1 Discharge Capacity Capacity (mAh/g) Retention Ratio 1st Cycle30th Cycle (%) Example 1-1 226 210 93 Comparative 203 166 82 Example 1-1Comparative 180 128 71 Example 1-2 Comparative 157 102 65 Example 1-3

As shown in Table 1, despite the adoption of “coprecipitation method” inboth of Example 1-1 and Comparative Example 1-1, there was a differencein charge-discharge cycle performance. Further, there was a differencein charge-discharge cycle performance between Example 1-1 adopting“coprecipitation method” and Comparative Examples 1-2, 1-3 adopting“solid state method”.

In order to investigate the causes, each of the lithium transition metalcomposite oxides in Example 1-1 and Comparative Examples 1-1 to 1-3 wererespectively used as a positive active material for a nonaqueouselectrolyte secondary battery, and a plurality of nonaqueous electrolytesecondary batteries were assembled by the above procedure. On allnonaqueous electrolyte secondary batteries assembled, initial charge wasperformed one time. The voltage control was all performed for a positiveelectrode potential. The condition of the initial charge was set to be0.1 CmA in a current, and the charge termination condition was set at atime point at which the current value decreased to 0.02 CmA. However,with respect to the charge voltage, different charge voltages areemployed for the batteries according to the same prescription. Forexample, in the case of Comparative Example 1-1, 9 batteries wereassembled, and for 8 batteries among these batteries, charge voltages of4.4 V, 4.5 V, 4.55 V, 4.6 V, 4.65 V, 4.7 V, 4.8 V and 5.0 V wererespectively employed.

Next, all batteries including one battery which was not subjected to theinitial charge were disassembled in a dry room, and the positiveelectrode plates were taken out. The positive electrode plates taken outfrom all batteries of Example 1 and Comparative Examples 2, 3 were stuckto a sample holder for measurement as-is without being washed and thelike, and subjected to X-ray diffraction measurement by an X-raydiffraction apparatus (manufactured by Rigaku Corporation, model number:MiniFlex II) using a CuKα radiation source. However, in each of thepositive electrode plates taken out from 9 batteries of ComparativeExample 1, a positive composite was collected removing an aluminum foilcurrent collector, and encapsulated into a tube made of Lindemann glass(manufactured by TOHO KK., length: 80 mm, outer diameter: 0.3 mm, innerdiameter: 0.1 mm) without being washed and the like. X-ray diffractionmeasurement of this sample for measurement was carried out by using alarge-scale synchrotron radiation facility SPring-8 (BL19). A wavelengthused was 0.7 Å. The results of measurement are shown in FIGS. 1 to 4.

In a series of X-ray diffraction charts of Example 1-1 shown in FIG. 1,focusing on the difference between diffraction patterns, the respectivecharts were traced in order from an upper side pattern indicating a lowpotential side pattern indicating to a lower side of a high potentialside, and consequently, a peak around a diffraction angle of 18°attributed to (003) line of a hexagonal crystal and a peak around adiffraction angle of 45° attributed to (104) line of a hexagonal crystalwere found to be shifted to a high angle side, but a split was not yetobserved when the potential reached 5.0 V. That is, in a series of X-raydiffraction charts of Example 1-1, the change in the diffraction patternattributed to the hexagonal crystal was not found, and a crystal phaseattributed to a cubic crystal did not appear even when electrochemicaloxidation stage proceeded to a potential of 4.8 V or more, and furtherreached a potential of 5.0 V. In addition, in this measurement, sincethe aluminum foil current collector was not removed from the measurementsample, a peak resulting from a metal aluminum was observed around 65°.

In a series of X-ray diffraction charts of Comparative Example 1-1 shownin FIG. 2, focusing on the difference between diffraction patterns, therespective charts were traced in order from an upper side patternindicating a low potential side to a lower side pattern indicating ahigh potential side, and consequently, a peak around a diffraction angleof 7° attributed to (003) line of a hexagonal crystal and a peak arounda diffraction angle of 20° attributed to (104) line of a hexagonalcrystal were found to be gradually shifted to a high angle side in thesamples of a potential of 4.6 V or more, and a split was clearlyobserved in the samples of potentials of 4.7 V and 4.8 V, and only apeak on the high angle side was observed in the sample of a potential of5.0 V. Further, two peaks observed around the diffraction angle of 26 to27° attributed to (108) line and (110) line of a hexagonal crystal weregradually shifted to come close to each other in the sample of potentialof 4.65 V or more, the peak on a low angle side began to disappear inthe samples of potentials of 4.7 V and 4.8 V, and one peak was observedin the sample of potential of 5.0 V. From this, it is presumed that inthe lithium transition metal composite oxide of Comparative Example 1-1,a crystal phase attributed to a cubic crystal appears in addition to acrystal phase attributed to a hexagonal crystal with the progress ofelectrochemical oxidation, and an oxidation reaction proceeds in a stateof coexisting two phases and the crystal phase attributed to a hexagonalcrystal is changed ultimately into the crystal phase attributed to acubic crystal.

X-ray diffraction charts in the case of adopting a potential of 5 V ofComparative Examples 1-2 and 1-3 are shown in FIG. 3A and FIG. 4A. Inboth charts, since it can be visually observed that a peak around adiffraction angle of 18° attributed to (003) line of a hexagonal crystalsplits, it is found that the crystal phase is not a single phase of ahexagonal crystal and in a state of coexistence with another phase inaddition to the hexagonal crystal. Just to make sure, enlarged views ofthe peaks are shown in FIG. 3B and FIG. 4B.

It has been found from the above findings that the active materialcontaining a lithium transition metal composite oxide, having a crystalstructure of an α-NaFeO₂ type, represented by a compositional formulaLi_(1+α)Me_(1−α)O₂ (Me is a transition metal element including Co, Niand Mn, α>0) and having a molar ratio Li/Me of Li to the all transitionmetal elements Me of 1.2 to 1.6, is characterized by being observed as asingle phase of a hexagonal crystal structure (a single phase attributedto a space group R3-m) on an X-ray diffraction chart when beingelectrochemically oxidized up to a potential of 5.0 V (vs. Li/Li⁺), andthereby the active material enables the charge-discharge cycleperformance of the nonaqueous electrolyte secondary battery using theactive material to be excellent.

In addition, since a common X-ray diffraction measurement apparatususing a CuKα radiation is used in the measurement of FIG. 1, and alarge-scale synchrotron radiation facility SPring-8 is used in themeasurement of FIG. 2, a diffraction angle, of diffraction peakattributed to a hexagonal crystal (attributed to a space group R3-m)appears is largely different between both measurement, but there is nodifference between both diffraction patterns and therefore similaranalysis can be performed.

Examples 1-2 to 1-61, Comparative Examples 1-4 to 1-16

A lithium transition metal composite oxide was synthesized in the samemanner as in Example 1-1 except for changing the mixing ratio betweenthe coprecipitated carbonate precursor and lithium carbonate and thesintering temperature according to descriptions shown in Tables 2 and 3.

Further, but not shown in Tables 2 and 3, a pH of the content in thereaction tank was controlled so as to be maintained always at 10.0(±0.05) in Example 1-25, always at 11.0 (±0.05) in Example 1-26, andalways at 8.6 (±0.05) as with Example 1-1 in other Examples andComparative Examples in the step of adding dropwise an aqueous sulfatesolution and an aqueous solution containing sodium carbonate and ammoniato produce a coprecipitated carbonate precursor.

All lithium transition metal composite oxides thus obtained weresubjected to X-ray diffraction measurement by an X-ray diffractionapparatus (manufactured by Rigaku Corporation, model number: MiniFlexII) using a CuKα radiation source, and consequently, an intensity ratioI₍₀₀₃₎/I₍₁₁₄₎ between the diffraction peak of (003) line and thediffraction peak of (114) line of all lithium transition metal compositeoxides in Examples 1-1 to 1-61 and Comparative Examples 1-4 to 1-16 was1.58 or more, as shown in Tables 2 and 3.

Further, the lithium transition metal composite oxides in Examples 1-2to 1-61 were also confirmed to be observed as a single phase attributedto a space group R3-m (a single phase of a hexagonal crystal structure)on an X-ray diffraction chart when the lithium transition metalcomposite oxide was electrochemically oxidized up to a potential of 5.0V (vs. Li/Li⁺).

Each of the lithium transition metal composite oxides in Examples 1-2 to1-61 and Comparative Examples 1-4 to 1-16 was used as a positive activematerial for a lithium secondary battery, and a lithium secondarybattery was assembled by the following procedure. A paste for applying,in which N-methylpyrrolidone was used as a dispersion medium and thepositive active material, acetylene black (AB) and polyvinylidenefluoride (PVdF) were kneaded/dispersed in a mass ratio of 90:5:5, wasprepared. The paste for applying was applied to one surface of analuminum foil current collector having a thickness of 20 μm to prepare apositive electrode plate. In addition, a mass of the active materialapplied per a certain area and an applied thickness were standardized sothat the respective test conditions were the same among all lithiumsecondary batteries of Examples and Comparative Examples. In this waypositive electrode plates of Examples 1-2 to 1-61 and ComparativeExamples 1-4 to 1-16 were prepared.

Nonaqueous electrolyte secondary batteries were assembled in the samemanner as in Example 1-1 except for using these positive electrodeplates.

(Initial Efficiency Test)

The lithium secondary batteries prepared by the above procedure weresubjected to an initial charge-discharge step at 25° C. The voltagecontrol was all performed for a positive electrode potential. Charge wasconstant current-constant voltage charge with a current of 0.1 CmA and avoltage of 4.6 V, and the charge termination condition was set at a timepoint at which the current value decreased to ⅙. Discharge was carriedout at constant current discharge under the conditions of a current of0.1 CmA and a final voltage of 2.0 V. This charge-discharge was carriedout by two cycles. Here, a rest process of 30 minutes was provided aftercharge and after discharge, respectively. A percentage represented by“(discharge electrical quantity)/(charge electrical quantity)×100” atthe first cycle in the initial charge-discharge step was recorded as“initial efficiency (%)”.

(Charge-Discharge Test)

Next, changing a charge voltage, a charge-discharge test of one cyclewas performed. The voltage control was all performed for a positiveelectrode potential. The conditions of the charge-discharge test werethe same as the conditions of the above initial charge-discharge stepexcept for setting the charge voltage to 4.3 V (vs. Li/Li⁺). Thedischarge electrical quantity at this time was recorded as “dischargecapacity (mAh/g)” (denoted by “0.1 C capa” in Table).

(High Rate Discharge Test)

Subsequently, a charge voltage was set to 4.3 V (vs. Li/Li⁺), charge wasperformed at a current of 0.1 CmA, and after rest of 30 minutes,discharge was performed at 2 CmA under the condition of a final voltageof 2.0 V. A percentage of the discharge capacity obtained at this timeto the “discharge capacity (mAh/g)” obtained at the time of 0.1 CmA wasdenoted by “2 C/0.1 C”.

(Measurement of Specific Surface Area)

An adsorbed amount [m²] of nitrogen on the active material wasdetermined by one point method using a specific surface area measurementapparatus manufactured by YUASA IONICS Co., Ltd. (trade name: MONOSORB).A value of the measured adsorbed amount [m²] divided by an activematerial mass (g) was considered as a BET specific surface area. Inmeasurement, gas adsorption through cooling using liquid nitrogen wascarried out. Further, the sample was preheated at 120° C. for 15 minutesprior to the cooling. An amount of the measurement sample loaded was0.5±0.01 g.

(Measurement of Tapped Density)

Using a tapping apparatus (made in 1968) manufactured by REI ELECTRICCO., LTD., a value of an active material volume after counted tapping of300 times divided by a mass of the active material was considered as atapped density. In measurement, 2g±0.2 g of the active material wasloaded into a 10⁻² dm³ measuring cylinder.

The results of the measurement test at the time when each of the lithiumtransition metal composite oxides of Examples 1-1 to 1-61 andComparative Examples 1-4 to 1-16 was used as a positive active materialfor a lithium secondary battery, are shown in Tables 2 and 3.

TABLE 2 Before Initial 0.1 C Ratio Ratio Ratio Sintering BET tapCharge-Discharge Efficiency capa 2 C/0.1 C Li/Me Co/Me Mn/Me Temperature(m²/g) (g/cm³) L₍₀₀₃₎/L₍₁₁₄₎ (%) (mAh/g) (%) Example 1-2 1.20 0.1250.688 900° C. 4.42 1.62 1.63 91.4% 184.7 53.2% Example 1-3 1.20 0.1250.688 850° C. 4.67 1.67 1.67 92.3% 183.5 55.1% Example 1-4 1.20 0.1250.688 800° C. 4.81 1.63 1.65 93.2% 183.7 55.7% Example 1-5 1.25 0.1250.688 900° C. 3.13 1.58 1.68 93.0% 212.9 80.1% Example 1-6 1.25 0.1250.688 850° C. 3.44 1.63 1.58 91.7% 209.4 78.5% Example 1-7 1.25 0.1250.688 800° C. 4.00 1.91 1.61 92.1% 208.5 79.9% Comparative 1.30 0.0000.688 900° C. 3.87 1.59 1.62 79.0% 183.6 57.0% Example 1-4 Comparative1.30 0.010 0.688 900° C. 3.77 1.61 1.59 79.0% 185.2 58.5% Example 1-5Example 1-8 1.30 0.020 0.688 900° C. 3.73 1.63 1.61 81.9% 197.6 71.1%Example 1-9 1.30 0.030 0.688 900° C. 3.66 1.62 1.63 83.9% 198.1 72.8%Example 1-10 1.30 0.040 0.688 900° C. 3.61 1.65 1.62 84.7% 199.2 73.9%Example 1-11 1.30 0.050 0.688 900° C. 3.54 1.65 1.61 84.4% 202.2 76.2%Example 1-12 1.30 0.058 0.688 900° C. 3.44 1.67 1.59 88.6% 210.4 77.1%Example 1-13 1.30 0.070 0.688 900° C. 3.53 1.68 1.68 89.0% 218.0 77.0%Example 1-14 1.30 0.080 0.688 900° C. 3.44 1.70 1.66 89.4% 219.5 76.5%Example 1-15 1.30 0.090 0.688 900° C. 3.38 1.71 1.63 89.9% 221.0 76.8%Example 1-16 1.30 0.100 0.688 900° C. 3.49 1.72 1.59 90.6% 222.0 76.3%Example 1-17 1.30 0.115 0.688 900° C. 3.22 1.73 1.64 91.1% 223.9 76.0%Comparative 1.30 0.125 0.615 900° C. 4.02 1.89 1.68 84.6% 167.3 55.5%Example 1-6 Example 1-18 1.30 0.125 0.625 900° C. 3.96 1.98 1.69 86.6%204.5 83.6% Example 1-19 1.30 0.125 0.650 900° C. 3.88 1.88 1.67 90.4%225.7 78.3% Example 1-20 1.30 0.125 0.661 900° C. 3.85 1.75 1.68 93.0%230.1 78.0% Example 1-21 1.30 0.125 0.673 900° C. 3.80 1.78 1.66 91.7%223.3 80.0% Example 1-22 1.30 0.125 0.684 900° C. 3.76 1.69 1.67 90.9%219.9 80.7% Example 1-1 1.30 0.125 0.688 900° C. 3.91 1.91 1.68 93.2%225.6 78.8% Example 1-23 1.30 0.125 0.688 850° C. 3.87 1.64 1.69 93.6%220.5 83.3% Example 1-24 1.30 0.125 0.688 800° C. 5.19 1.92 1.62 93.9%224.4 81.5% Example 1-25 1.30 0.125 0.688 900° C. 5.58 0.58 1.61 88.5%211.6 62.3% Example 1-26 1.30 0.125 0.688 900° C. 5.87 0.52 1.63 84.3%205.2 58.9% Example 1-27 1.30 0.125 0.696 900° C. 3.77 1.66 1.63 86.2%208.3 73.3% Example 1-28 1.30 0.125 0.707 900° C. 3.62 1.44 1.60 85.2%203.1 73.1% Example 1-29 1.30 0.125 0.719 900° C. 3.65 1.25 1.63 82.1%200.5 72.3% Comparative 1.30 0.125 0.725 900° C. 3.65 1.75 1.62 78.9%162.5 54.5% Example 1-7 Example 1-30 1.30 0.135 0.688 900° C. 3.78 1.901.62 92.0% 224.0 75.6% Example 1-31 1.30 0.145 0.688 900° C. 3.42 1.851.64 91.5% 218.0 74.5% Example 1-32 1.30 0.155 0.688 900° C. 3.28 1.881.68 91.0% 214.0 74.9% Example 1-33 1.30 0.165 0.688 900° C. 3.35 1.861.61 90.4% 210.0 74.2% Example 1-34 1.30 0.173 0.688 900° C. 2.89 1.841.63 88.5% 206.4 73.2% Example 1-35 1.30 0.185 0.688 900° C. 2.92 1.861.64 88.0% 205.0 73.0% Example 1-36 1.30 0.195 0.688 900° C. 3.02 1.901.65 87.5% 204.5 72.8% Example 1-37 1.30 0.205 0.688 900° C. 2.89 1.911.63 87.0% 204.0 72.9% Example 1-38 1.30 0.215 0.688 900° C. 2.82 1.881.67 83.5% 200.5 71.7% Example 1-39 1.30 0.223 0.688 900° C. 2.99 1.871.66 83.0% 200.3 71.6% Example 1-40 1.30 0.230 0.688 900° C. 2.77 1.921.63 82.8% 200.2 71.8% Comparative 1.30 0.240 0.688 900° C. 2.79 1.901.59 75.5% 176.3 68.3% Example 1-8

TABLE 3 Before Initial 0.1 C Ratio Ratio Ratio Sintering BET tapCharge-Discharge Efficiency capa 2 C/0.1 C Li/Me Co/Me Mn/Me Temperature(m²/g) (g/cm³) L₍₀₀₃₎/L₍₁₁₄₎ (%) (mAh/g) (%) Comparative 1.30 0.2500.688 900° C. 2.83 1.88 1.63 69.7% 147.9 62.4% Example 1-9 Comparative1.30 0.260 0.688 900° C. 2.85 1.90 1.64 65.8% 123.4 58.3% Example 1-10Comparative 1.30 0.270 0.688 900° C. 2.91 1.91 1.62 62.3% 119.7 50.7%Example 1-11 Comparative 1.30 0.280 0.688 900° C. 2.82 1.91 1.62 58.4%111.3 42.3% Example 1-12 Comparative 1.30 0.288 0.688 900° C. 2.89 1.911.63 55.7% 105.4 36.2% Example 1-13 Comparative 1.35 0.125 0.615 900° C.3.76 1.90 1.62 82.4% 166.5 56.5% Example 1-14 Example 1-41 1.35 0.1250.650 900° C. 3.75 1.87 1.63 83.5% 213.2 77.7% Example 1-42 1.35 0.1250.661 900° C. 3.71 1.85 1.61 87.2% 228.0 77.3% Example 1-43 1.35 0.1250.673 900° C. 3.68 1.81 1.64 89.2% 226.6 79.2% Example 1-44 1.35 0.1250.684 900° C. 3.65 1.76 1.66 88.0% 223.0 75.4% Example 1-45 1.35 0.1250.688 900° C. 2.98 1.88 1.62 91.3% 231.6 79.5% Example 1-46 1.35 0.1250.688 850° C. 3.79 1.91 1.65 90.2% 215.8 81.6% Example 1-47 1.35 0.1250.688 800° C. 3.71 1.66 1.64 90.4% 221.4 76.4% Example 1-48 1.35 0.1250.696 900° C. 3.62 1.64 1.66 86.6% 213.5 77.7% Example 1-49 1.35 0.1250.707 900° C. 3.59 1.55 1.68 86.8% 208.5 78.3% Example 1-50 1.35 0.1250.719 900° C. 3.54 1.39 1.64 82.0% 204.5 74.3% Comparative 1.35 0.1250.725 900° C. 3.54 1.78 1.63 77.9% 162.5 52.7% Example 1-15 Example 1-511.40 0.125 0.688 1000° C.  0.51 1.86 1.68 72.2% 199.8 33.7% Example 1-521.40 0.125 0.688 980° C. 0.64 1.88 1.68 75.1% 201.4 42.5% Example 1-531.40 0.125 0.688 960° C. 0.75 1.87 1.88 77.1% 203.2 54.2% Example 1-541.40 0.125 0.688 940° C. 0.88 1.85 1.68 80.5% 216.0 60.8% Example 1-551.40 0.125 0.688 920° C. 1.05 1.87 1.68 81.8% 217.8 69.2% Example 1-561.40 0.125 0.688 900° C. 1.24 1.88 1.69 82.5% 223.0 76.8% Example 1-571.40 0.125 0.688 850° C. 2.25 1.67 1.67 83.3% 212.9 80.1% Example 1-581.40 0.125 0.688 800° C. 3.43 1.89 1.68 82.8% 206.9 76.0% Example 1-591.45 0.125 0.688 900° C. 0.86 1.88 1.66 77.3% 188.2 65.3% Example 1-601.50 0.125 0.688 1000° C.  0.51 1.61 1.69 75.2% 223.6 52.0% Example 1-611.60 0.125 0.688 1000° C.  0.32 1.42 1.81 69.0% 204.2 51.0% Comparative1.80 0.125 0.688 1000° C.  0.67 1.81 1.83 56.6% 116.9 52.4% Example 1-16

From Tables 2 to 3, it is found that a high discharge capacity can beobtained when the lithium transition metal composite oxide, in which theratio Li/Me is 1.2 to 1.6, the ratio Co/Me is 0.02 to 0.23, and theratio Co/Me is 0.62 to 0.72, is used as a positive active material for alithium secondary battery. It is found that among these, when the ratioLi/Me is 1.25 to 1.40, a lithium secondary battery having a highdischarge capacity and excellent initial efficiency can be obtained, andwhen the ratio Li/Me is 1.250 to 1.350, the ratio Co/Me is 0.040 to0.195, and the ratio Mn/Me is 0.625 to 0.707, the initial efficiency isfurther improved, and high rate discharge performance is also improved.It can be said that a BET specific surface area is preferably 0.88 m²/gor more in order to improve the initial efficiency and the high ratedischarge performance further, and a tapped density is preferably 1.25g/cm³ or more in order to improve the high rate discharge performancefurther.

Example 2 Example 2-1

4.6959 g of cobalt sulfate heptahydrate, 7.0043 g of nickel sulfatehexahydrate and 21.766 g of manganese sulfate pentahydrate were weighed,and dissolved in 200 ml of ion-exchanged water to prepare a 0.67 Maqueous sulfate solution of which the molar ratio of Co:Ni:Mn was12.5:18.75:68.75. On the other hand, 750 ml of ion-exchanged water waspoured into a 2 dm³ reaction tank, and CO₂ was dissolved in theion-exchanged water by bubbling CO₂ gas for 30 minutes in theion-exchanged water. A temperature of the reaction tank was set to 50°C. (±2° C.), and the aqueous sulfate solution was added dropwise at arate of 3 ml/min while stirring the content in the reaction tank at arotational speed of 700 rpm using a paddle blade equipped with astirring motor. Here, from the start of the dropwise addition until thecompletion thereof, an aqueous solution containing 0.67 M of sodiumcarbonate and 0.067 M of ammonia was appropriately added dropwise tocontrol the content in the reaction tank so as to always maintain a pHof 8.6 (±0.05). After the completion of the dropwise addition, thestirring was continued for further one hour. After the stirring wasstopped, the solution was left standing for 12 hours or more.

Next, using a suction filtration apparatus, particles of coprecipitatedcarbonate salt produced in the reaction tank were separated, and sodiumions adhering to the particles were cleaned and removed withion-exchange water, and the resulting particles were dried at 100° C.under ordinary pressure in an air atmosphere by using an electricfurnace. Thereafter, particles were pulverized for several minutes byusing an automatic mortar made of agate in order to level particlesizes. In this way, a coprecipitated carbonate precursor was prepared.

Lithium carbonate (0.9699 g) was added to 2.2780 g of the coprecipitatedcarbonate precursor, and the resulting mixture was adequately mixed byusing an automatic mortar made of agate to prepare a mixed powder inwhich the molar ratio of Li and (Co, Ni, Mn) was 130:100.

The mixed powder was molded at a pressure of 6 MPa by using a pelletmolding machine to be formed into pellets with a diameter of 25 mm. Theamount of the mixed powder subjected to pellet molding was determined soas to be equivalent to 2 g of a mass of an assumed final product. One ofthe pellets was placed on an aluminum boat having an entire length ofabout 100 mm, placed together with the boat in a box type electricfurnace (model number: AMF 20), and sintered at 900° C. for 10 hoursunder ordinary pressure in an air atmosphere. Inner dimensions of thebox type electric furnace were 10 cm long, 20 cm wide and 30 cm deep,and heating wires were disposed in a width direction at 20-centimeterintervals. After sintering, a heater was turned off, and the aluminumboat was naturally cooled as it was left standing in the furnace.Consequently, a temperature of the furnace was lowered to about 200° C.after 5 hours, but a subsequent temperature lowering rate was slightlymild. After a lapse of a whole day and night, it was confirmed that atemperature of the furnace was 100° C. or lower, and then the pellet wastaken out and pulverized for several minutes by using an automaticmortar made of agate in order to level particle sizes. In this way, alithium transition metal composite oxide of Example 2-1 was prepared.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.13)Co_(0.11)Ni_(0.17)Mn_(0.59)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation, the lithiumtransition metal composite oxide was observed to have a single phase ofa hexagonal crystal structure of an α-NaFeO₂ type.

Example 2-2

A lithium transition metal composite oxide of Example 2-2 was preparedby the same procedure as in Example 2-1 except for using, as a mixedpowder to be subjected to pellet-molding, a mixed powder, in which themolar ratio of Li and (Co, Ni, Mn) was 140:100, prepared by adding1.0216 g of lithium carbonate to 2.2278 g of the coprecipitatedcarbonate precursor prepared in Example 2-1, and adequately mixing theresulting mixture by using an automatic mortar made of agate.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.17)Co_(0.10)Ni_(0.17)Mn_(0.56)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation, the lithiumtransition metal composite oxide was observed to have a single phase ofa hexagonal crystal structure of an α-NaFeO₂ type.

Comparative Example 2-1

A reaction tank used in the present comparative example was cylindrical,and was provided with an overflow pipe for always ejecting a slurry of areaction crystallization product out of a system at a certain flow ratein the upper part, and had a volume of 5 L. Pure water (4 L) was put inthe reaction tank. Further, a 32% aqueous sodium hydroxide solution wasadded so as to have a pH of 11.6. The aqueous sodium hydroxide solutionwas stirred at a rotational speed of 1350 rpm using a stirrer equippedwith a stirring blade of a paddle type, and a solution temperature inthe reaction tank was maintained at 50° C. by a heater.

An aqueous solution of nickel sulfate (NiSO₄) having a concentration of1.0 mold, an aqueous solution of manganese sulfate (MnSO₄) having aconcentration of 1.0 mold, an aqueous solution of cobalt sulfate (CoSO₄)having a concentration of 1.0 mold, an aqueous ammonium sulfate((NH₄)₂SO₄) solution having a concentration of 6 mold, and a 4 wt %aqueous hydrazine (NH₂NH₂) solution were respectively mixed in a volumeratio of 0.33:0.33:0.33:0.05:0.01 to prepare an aqueous sulfate solutionin which the molar ratio of Co, Ni, and Mn was 1:1:1.

The aqueous sulfate solution was continuously added dropwise to thereaction tank at a flow rate of 13 mL/min. A 32% aqueous sodiumhydroxide solution was intermittently loaded into the reaction tank soas to have a constant pH of 11.3. A solution temperature in the reactiontank was intermittently controlled by a heater so as to be constant at50° C.

A slurry of a Ni—Mn—Co composite oxide as a reaction crystallizationproduct was continuously collected from the overflow pipe for 24 hoursafter a elapse of 50 hours from the start of loading a raw materialsolution. The collected slurry was washed with water and filtrated. Thefiltrated substance was dried at 100° C. for 20 hours to obtain a driedpowder of a coprecipitated hydroxide precursor.

The obtained coprecipitated hydroxide precursor and a lithium hydroxidemonohydrate salt powder were weighed so as to have the molar ratio of Liand (Co, Ni, Mn) of 102:100, and these compounds were adequately mixed.The mixture was filled in an aluminum sagger, and a temperature of themixture was raised to 1000° C. at a temperature raising rate of 100°C./h, maintained at 1000° C. for 15 hours and cooled to 600° C. at atemperature lowering rate of 100° C./h using the electric furnace undera dry air stream, and then the mixture was left standing to be cooled.In this way, a lithium transition metal composite oxide of ComparativeExample 2-1 was obtained.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.0)C_(0.03)Ni_(0.33)Mn_(0.33)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation source, thelithium transition metal composite oxide was observed to have a singlephase of a hexagonal crystal structure of an α-NaFeO₂ type.

Comparative Example 2-2

An aqueous sulfate solution, in which the respective elements Co, Ni,and Mn were dissolved in the molar ratio of 12.5:18.75:68.75, wasprepared. On the other hand, a temperature of a reaction tank pouredwith ion-exchange water was maintained at 50° C., and an aqueous NaOHsolution was added dropwise thereto to adjust a pH to 11.5. Next,dissolved oxygen was removed by bubbling an inert gas. The aqueoussulfate solution was added dropwise at a feed rate of 3 ml/min whilestirring the content in the reaction tank. Simultaneously, an aqueoushydrazine solution as a reducing agent was added dropwise at a feed rateof 0.83 ml/min. While the dropwise addition was continued, thetemperature of the reaction tank was maintained at 50° C., and anaqueous NaOH solution was appropriately added dropwise while monitoringa pH so that the pH always falls within the range of 11.5±0.05. Afterthe completion of the dropwise addition, the stirring was stopped andthe solution was left standing for 12 hours or more. Next, the resultingcoprecipitated product was separated by filtration and dried at 100° C.under ordinary pressure in an air atmosphere by using an oven. Afterdrying, the coprecipitated product was pulverized lightly to an extentof leveling particle sizes. Thereby, a dried powder was obtained.

Lithium hydroxide was added to the obtained dried powder so as to havethe molar ratio of Li and (Co+Ni+Mn) of 150:100, and the resultingmixture was dry-mixed to prepare a mixed powder. Next, 5 kg of the mixedpowder was placed in an electric furnace and sintered under ordinarypressure in an air atmosphere at 1000° C. over 12 hours. Aftersintering, a heater was turned off, and the powder was naturally cooledas it was left standing in the furnace. After a elapse of a whole dayand night, it was confirmed that a temperature of the furnace was 100°C. or lower, and then the powder was taken out and pulverized lightly toan extent of leveling particle sizes. In this way, a lithium transitionmetal composite oxide of Comparative Example 2-2 was obtained.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.2)Co_(0.1)Ni_(0.15)Mn_(0.55)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation source, thelithium transition metal composite oxide was observed to have a singlephase of a hexagonal crystal structure of an α-NaFeO₂ type.

Comparative Example 2-3

A lithium transition metal composite oxide of Comparative Example 2-3was prepared by the same procedure as in Example 2-1 except forperforming sinter at 1000° C. for 10 hours as a sintering condition.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.13)Co_(0.11)Ni_(0.17)Mn_(0.59)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation, the lithiumtransition metal composite oxide was observed to have a single phase ofa hexagonal crystal structure of an α-NaFeO₂ type.

Comparative Example 2-4

A coprecipitated hydroxide precursor was prepared by the same procedureas in Comparative Example 1 except for using an aqueous sulfate solutionin which the molar ratio of Co, Ni, and Mn was 12.5:19.94:67.56. Next,in a sintering step, a lithium transition metal composite oxide ofComparative Example 2-4 was prepared by employing the same procedure asin Example 2-1 except for preparing a mixed powder in which the molarratio of Li and (Co,Ni,Mn) was 130:100 by using the obtainedcoprecipitated hydroxide salt precursor and a lithium hydroxidemonohydrate salt powder.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.13)Co_(0.11)Ni_(0.17)Mn_(0.59)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation, the lithiumtransition metal composite oxide was observed to have a single phase ofa hexagonal crystal structure of an α-NaFeO₂ type.

Comparative Example 2-5

Lithium carbonate (Li₂CO₃), cobalt hydroxide (Co(OH)₂), nickel hydroxide(Ni(OH)₂) and manganese oxyhydroxide (MnOOH) were weighed so as to havethe ratio of 130:12.50:19.94:67.56 of the respective elements Li, Co,Ni, and Mn, and these raw materials were adequately mixed and pulverizedusing a mortar to obtain a raw material mixture. Next, 3 g of a rawmaterial mixture was take out from the raw material mixture, andsintered at 900° C. for 10 hours in the air. In this way, a lithiumtransition metal composite oxide of Comparative Example 2-5 wasobtained.

As a result of composition analysis, the obtained lithium transitionmetal composite oxide had the composition ofLi_(1.13)Co_(0.11)Ni_(0.17)Mn_(0.59)O₂. Further, as a result of powderX-ray diffraction measurement using a CuK_(α) radiation, the lithiumtransition metal composite oxide was observed to have a single phase ofa hexagonal crystal structure of an α-NaFeO₂ type.

(Assembling and Evaluation of Nonaqueous Electrolyte Secondary Battery)

Each of the lithium transition metal composite oxides in Examples 2-1,2-2 and Comparative Examples 2-1 to 2-5 was used as a positive activematerial for a nonaqueous electrolyte secondary battery, and anonaqueous electrolyte secondary battery was assembled by the followingprocedure, and battery characteristics thereof were evaluated.

A positive active material, acetylene black (AB) and polyvinylidenefluoride (PVdF) were respectively mixed in a mass ratio of 85:8:7. Tothis, N-methylpyrrolidone as a dispersion medium was added, and theresulting mixture was kneaded/dispersed to prepare a applying paste. Inaddition, a mass ratio of the PVdF was shown on a solid mass basis sincea liquid in which a solid content was dissolved/dispersed was used. Theapplying paste was applied onto an aluminum foil current collectorhaving a thickness of 20 μm to prepare a positive electrode plate.

A lithium metal was used for a counter electrode (negative electrode) inorder to observe the behavior of the positive electrode alone. Thelithium metal was closely attached to a nickel foil current collector.However, it was prepared in such a manner that the capacity of thenonaqueous electrolyte secondary battery was controlled adequately bythe positive electrode.

As an electrolyte solution, a solution obtained by dissolving LiPF₆, soas to have a concentration of 1 mol/l, in a mixed solvent in which avolume ratio of EC/EMC/DMC was 6:7:7 was used. A microporous membranemade of polypropylene, in which an electrolyte retainbility was improvedby surface modification using polyacrylate, was used as a separator. Anickel plate, to which a lithium metal foil was attached, was used as areference electrode. A metal-resin composite film made of polyethyleneterephthalate (15 μm)/aluminum foil (50 μm)/metal-adhesive polypropylenefilm (50 μm) was used for a outer case. The respective electrodes werehoused in the outer case in such a way that open ends of a positiveelectrode terminal, a negative electrode terminal and a referenceelectrode terminal were exposed to outside. And fusion margins with theinner surfaces of the aforementioned metal resin composite films facingeach other were airtightly sealed except a portion forming anelectrolyte solution filling hole.

On the nonaqueous electrolyte secondary batteries thus assembled, aninitial charge-discharge step of two cycles was performed at 25° C. Thevoltage control was all performed for a positive electrode potential.Charge was constant current-constant voltage charge with a current of0.1 CmA and a voltage of 4.6 V. The charge termination condition was setat a time point at which the current value decreased to 0.02 CmA.Discharge was carried out at constant current discharge under theconditions of a current of 0.1 CmA and a final voltage of 2.0 V. In allcycles, a rest time of 30 minutes was set after charge and afterdischarge. Nonaqueous electrolyte secondary batteries of Examples andComparative Examples were completed in this way.

On the completed nonaqueous electrolyte secondary batteries,charge-discharge of three cycles was performed. The voltage control wasall performed for a positive electrode potential. The conditions of thecharge-discharge cycle test were the same as the conditions of the aboveinitial charge-discharge step except for setting the charge voltage to4.3 V (vs. Li/Li⁺). In all cycles, a rest time of 30 minutes was setafter charge and after discharge.

Next, a high rate discharge test was carried out by the followingprocedure. First, constant current and constant voltage charge in whicha current was 0.1 CmA and a voltage was 4.3 V was carried out. Afterrest of 30 minutes, constant current discharge under the conditions of acurrent of 1 CmA and a final voltage of 2.0 V was performed, and adischarge capacity at this time was recorded as “high rate dischargecapacity (mAh/g)”.

(Measurement of Oxygen Position Parameter)

The battery after subjecting to the high rate discharge test was furthersubjected to additional discharge under the condition of constantcurrent discharge under the conditions of a current of 0.1 CmA and afinal voltage of 2.0 V, and then the positive electrode plate was takenout of the battery outlet case in a dry room. The positive electrodeplate taken out was subjected to X-ray diffraction measurement with apositive composite layer to the current collector without being washedand the like. Crystal structure analysis by a Rietveld method wasperformed on all diffracted lines excluding peaks resulting fromaluminum used as a metal foil current collector. As a software used forRietveld analysis, RIETAN 2000 (Izumi et al., Mater. Sci. Forum,321-324, p. 198 (2000)) was used. As a profile function used foranalysis, a pseudo-Voigt function of TCH was used. A peak position shiftparameter was previously refined by using a silicon standard sample(Nist 640c) having a known lattice constant. A crystal structure modelof the positive active material is set to a space group R3-m, and thefollowing parameters were refined at each atom position.

-   -   background parameter    -   lattice constant    -   oxygen position parameter z    -   half width parameter of Gauss function    -   half width parameter of Lorentz function    -   asymmetry parameter    -   preferred-orientation parameter    -   isotropic atomic displacement parameter (however, fixed to 0.75        for Li atom)

Diffraction data between 15° and 85° (CuKα) was used as actual data, andthis was refined to such an extent that an value of S indicating thedegree of difference from the crystal model structure was reduced below1.3.

Aside from the batteries subjected to the above-mentioned tests, each ofthe lithium transition metal composite oxides in Examples 2-1, 2-2 andComparative Examples 2-1 to 2-5 was used as a positive active materialfor a nonaqueous electrolyte secondary battery, and each nonaqueouselectrolyte secondary battery was assembled by the above procedure, andinitial charge was performed. The voltage control was all performed fora positive electrode potential. The condition of the initial charge wasset to the constant voltage and constant current charge under theconditions of a voltage of 5.0 V and a current of 0.1 CmA, and thecondition of ending the charge was set to be the time point when theelectric current value was decreased to 0.02 CmA.

These batteries were disassembled in a dry room, and the positiveelectrode plates were taken out. Each of the positive electrode platestaken out was stuck to a sample holder for measurement as-is withoutbeing washed and the like, and subjected to X-ray diffractionmeasurement by an X-ray diffraction apparatus (manufactured by RigakuCorporation, model number: MiniFlex II) using a CuKα radiation source.The results of measurement are shown in FIG. 5.

A series of charts on a left side of FIG. 5 are diffraction charts drawnwithin a diffraction angle range from 15° to 50°, and a series of chartson a right side are diffraction charts redrawn by setting a diffractionangle range to from 15° to 25° in order to observe a diffraction peakaround 19° in detail. From these diffraction charts, it is found thateach of the lithium transition metal composite oxides has a single phaseattributed to a space group R3-m in Examples 2-1, 2-2 and ComparativeExamples 2-1 to 2-4. On the other hand, in Comparative Example 2-5, itis found that as is characterized in that the diffraction peak around19° is split, a crystal structure belongs to R3-m, but it has amultiphase.

The results of the measurement of a high rate discharge capacity, theresults of the measurement of a oxygen position parameter, and theresults of the measurement of X-ray diffraction after electrochemicallyoxidizing up to a potential of 5.0 V (vs. Li/Li⁺) are shown in Table 4.

TABLE 4 Oxygen High Rate Position Discharge Phase (oxidized ParameterCapacity Composition Ratio Li/Me up to 5.0 V) (Discharge End) (mAh/g)Example 2-1 Li_(1.13)Co_(0.11)Ni_(0.17)Mn_(0.59)O₂ 1.3 Single Phase0.259 180 Example 2-2 Li_(1.17)Co_(0.10)Ni_(0.17)Mn_(0.56)O₂ 1.4 SinglePhase 0.260 182 Comparative LiCo_(0.33)Ni_(0.33)Mn_(0.33)O₂ 1.0 SinglePhase 0.263 142 Example 2-1 ComparativeLi_(1.20)Co_(0.10)Ni_(0.15)Mn_(0.55)O₂ 1.5 Single Phase 0.263 155Example 2-2 Comparative Li_(1.13)Co_(0.11)Ni_(0.17)Mn_(0.59)O₂ 1.3Single Phase 0.264 149 Example 2-3 ComparativeLi_(1.13)Co_(0.11)Ni_(0.17)Mn_(0.59)O₂ 1.3 Single Phase 0.265 148Example 2-4 Comparative Li_(1.13)Co_(0.11)Ni_(0.17)Mn_(0.59)O₂ 1.3Multiphase 0.260 135 Example 2-5

It has been found from the above knowledge that the active material fora nonaqueous electrolyte secondary battery containing a lithiumtransition metal composite oxide, having a crystal structure of anα-NaFeO₂ type, represented by a compositional formula Li_(1+α)Me_(1−α)O₂(Me is a transition metal element including Co, Ni and Mn, α>0) andhaving a compositional ratio (1+α)/(1−α) of Li to the transition metalelement Me of 1.2 to 1.6, is observed as a single phase attributed to aspace group R3-m on an X-ray diffraction chart when the lithiumtransition metal composite oxide is electrochemically oxidized up to apotential of 5.0 V (vs. Li/Li⁺) and is characterized in that an oxygenposition parameter determined by crystal structure analysis by aRietveld method based on an X-ray diffraction pattern is 0.260 or less,and thereby the active material enables the high rate dischargeperformance of the nonaqueous electrolyte secondary battery using theactive material to be excellent.

The reason why, with respect to lithium transition metal composite oxideobserved as a single phase attributed to a space group R3-m on an X-raydiffraction chart when the lithium transition metal composite oxide iselectrochemically oxidized up to a potential of 5.0 V (vs. Li/Li⁺), thelithium transition metal composite oxide having an oxygen positionparameter of 0.260 or less is more superior in the high rate dischargeperformance than the lithium transition metal composite oxide having anoxygen position parameter exceeding 0.260, is not necessarily clear.However, the present inventor presumes that since having a smaller valueof the oxygen position parameter means that an O (oxygen) position ismore apart from a Li (lithium) position, this may be linked with thefact that Li hardly undergoes an interaction with an oxygen atom inelectrochemically intercalation/deintercalation of Li.

Example 3

(Synthesis of Active Material)

A 2 M aqueous sulfate solution was prepared by weighing cobalt sulfateheptahydrate, nickel sulfate hexahydrate and manganese sulfatepentahydrate so as to have the molar ratio of 12.5:19.94:67.56 of Co,Ni, and Mn, and dissolving these compound in ion-exchanged water. On theother hand, a 15 L reaction tank was prepared. An outlet was fixed inthis reaction tank so that a solution was ejected from the outlet if aliquid level in the reaction tank exceeded a certain height. Further, astirring blade was fixed in the reaction tank, and a cylindricalconvection plate was fixed in the reaction tank in order to causeconvection in a vertical direction during stirring the solution.Ion-exchanged water (7 L) was poured into the reaction tank, and a CO₂gas was adequately dissolved in the ion-exchanged water by bubbling theCO₂ gas for 30 minutes in the ion-exchanged water. In addition, the CO₂gas bubbling was continued until the dropwise addition of the aqueoussulfate solution was completed. Next, a temperature of the reaction tankwas set to 50° C., and the mixing blade was operated at a rotationalspeed of 1000 rpm. The aqueous sulfate solution (2 L) was added dropwisegradually to the content in the reaction tank. The stirring wascontinued during the dropwise addition. Further, an aqueous solutioncontaining 2 M of sodium carbonate and 0.2 M of ammonia dissolved wasappropriately added while always monitoring a pH in the reaction tank soas to maintain the pH within the range of 8.6±0.2. A part of a solutioncontaining a reaction product was ejected out of the reaction tank fromthe liquid outlet during the aqueous sulfate solution was addeddropwise, but the ejected solution was discarded without being returnedto the reaction tank before the dropwise addition of all of 2 L of theaqueous sulfate solution was completed. After the completion of dropwiseaddition, a coprecipitated product was separated from a solutioncontaining a reaction product by suction filtration, and thecoprecipitated product was washed with ion-exchange water in order toremove sodium ions attaching to the coprecipitated product. Next, thecoprecipitated product was dried at 100° C. under ordinary pressure inan air atmosphere by using an oven. After drying, the coprecipitatedproduct was pulverized for several minutes by using a mortar in order tolevel particle sizes. In this way, a powder of a coprecipitatedcarbonate precursor was prepared.

Lithium carbonate was added to the above coprecipitated carbonateprecursor to prepare a mixed powder in which the molar ratio of Li andMe (Co, Ni, Mn) was 130:100. Here, the amount of lithium carbonate wasadjusted so that the amount of Li was 3% excessive relative to astoichiometric proportion. The mixed powder was transferred to a sagger,and placed in a furnace. A temperature of the furnace was raised fromroom temperature to 900° C. over 4 hours, and the mixed powder wassintered at 900° C. for 10 hours under ordinary pressure. After atemperature of the furnace was returned to ordinary temperature, asintered product was taken out and pulverized to an extent of levelingparticle sizes by using a mortar. In this way,Li[Li_(0.13)Co_(0.109)Ni_(0.173)Mn_(0.588)]O₂ (ratio Li/Me: 1.30) wasprepared.

(Assembling of Prismatic Lithium Secondary Battery)

FIG. 7 is a schematic sectional view of a prismatic lithium secondarybattery used in the present example. The prismatic lithium secondarybattery 1 is formed by housing a electric power generating elementincluding a flat rolled electrode group 2 formed by winding a positiveelectrode plate 3 in which a aluminum foil current collector is providedwith a positive composite layer containing a positive active material,and a negative electrode plate 4 in which a copper foil currentcollector is provided with a negative composite layer containing anegative active material with a separator 5 sandwiched between theelectrode plates, and a nonaqueous electrolyte containing electrolytesalt in a battery case 6 of 34 mm wide, 50 mm high and 5.2 mm thick.

A battery lid 7 provided with a safety valve 8 is attached to thebattery case 6 by laser welding, the negative electrode plate 4 isconnected a negative electrode terminal 9 through a negative electrodelead 11, and the positive electrode plate 3 is connected the battery lidthrough a positive electrode lead 10.

(Positive Electrode Plate)

The Li[Li_(0.13)Co_(0.109)Ni_(0.173)Mn_(0.588)]O₂ prepared in the aboveway was used as a positive active material, and a prismatic lithiumsecondary battery was prepared by the following procedure.

A positive paste, in which N-methylpyrrolidone was used as a dispersionmedium, and the positive active material, acetylene black (AB) andpolyvinylidene fluoride (PVdF) were kneaded/dispersed in a mass ratio of90:5:5, was prepared. The positive paste was applied onto both surfacesof an aluminum foil current collector having a thickness of 15 μm anddried. Next, a positive electrode plate was prepared by roller pressingso that a packed density of a composite is 2.6 g/cm³.

(Negative Electrode Plate)

On the other hand, a negative paste, in which ion-exchange water wasused as a dispersion medium and graphite as a negative active material,and carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR)were kneaded/dispersed in a mass ratio of 97:2:1, was prepared. Thenegative paste was applied onto both surfaces of a copper foil currentcollector having a thickness of 10 μm and dried. Next, a negativeelectrode plate was prepared by roller pressing so that a packed densityof a composite was 1.4 g/cm³.

(Electrolyte Solution)

As an electrolyte solution, a solution obtained by dissolving LiPF₆ soas to have a concentration of 1 mol/l in a mixed solvent in whichethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in avolume ratio of 3:7 was used.

(Separator)

A microporous membrane made of polyethylene (H6022 produced by AsahiKasei Corp.) having a thickness of 20 μm was used for a separator.

The prismatic lithium secondary battery (battery 1) assembled by theabove procedure was subjected to the following tests.

(Measurement of Battery Thickness)

A central part of a longer side of the assembled prismatic lithiumsecondary battery was sandwiched between calipers in a directionperpendicular to the longer side (a horizontal direction from a shortside) to measure a battery thickness. A measured value at this time wasrecorded as “a battery thickness (mm) before a test”.

Further, the measurement of the battery thickness was performed again inthe same manner as in the above after the following discharge capacitytest. A measured value at this time was recorded as “a battery thickness(mm) after a test”.

(Discharge Capacity Test)

First, an initial charge-discharge (first charge-discharge) of one cyclewas performed at 25° C. Charge was constant current-constant voltagecharge with a current of 0.1 CmA and a voltage of 4.6 V. Here, chargewas carried out at a constant current-constant voltage charge with acurrent of 0.2 CmA and a voltage of 4.5 V for 8 hours, and discharge wascarried out at constant current discharge under the conditions of acurrent of 0.2 CmA and a final voltage of 2.0 V. Subsequently, adischarge capacity test was performed. The conditions of the dischargecapacity test were composed of charge-discharge of one cycle under thesame conditions as in the above first charge-discharge except forchanging the charge voltage to 4.2 V. A discharge electrical quantity atthis time was recorded as “discharge capacity (mAh)”.

(Gas Analysis)

A battery after discharge was disassembled in liquid paraffin, and allof gas released from the battery inside was collected in a manner ofwater substitution. Component analysis of the gas was carried out byusing gas chromatography (HP5890 series II gas chromatographmanufactured by HEWLETT PACKARD CORPORATION) equipped withMolecularSieve13X and Porapak Q columns (both manufactured by SPELCOGbR).

(Battery 2)

A discharge capacity test and gas analysis were performed in the samemanner as in the battery 1 except for setting the charge voltage at thefirst charge-discharge to 4.45 V using a lithium secondary battery(battery 2) assembled by the same procedure as in the battery 1.

(Battery 3)

A discharge capacity test and gas analysis were performed in the samemanner as in the battery 1 except for setting the charge voltage at thefirst charge-discharge to 4.40 V using a lithium secondary battery(battery 3) assembled by the same procedure as in the battery 1.

(Battery 4)

A discharge capacity test was performed in the same manner as in thebattery 1 except for setting the charge voltage at the firstcharge-discharge to 4.20 V using a lithium secondary battery (battery 4)assembled by the same procedure as in the battery 1.

The results of battery thickness measurement and the results of thedischarge capacity tests of the batteries 1 to 4 are shown in Table 5.

TABLE 5 Battery Charge Voltage Thickness (mm) Discharge Battery PositiveBefore After Capacity Battery Voltage (V) Electrode (V) Test Test (mAh)Battery 1 4.50 4.60 5.18 5.83 650 Battery 2 4.45 4.55 5.18 5.18 657Battery 3 4.40 4.50 5.18 5.18 576 Battery 4 4.20 4.30 8.18 5.18 247

The results of analysis of gas components in cells of the batteries 1, 2and 3 are shown in Table 6.

TABLE 6 Collected CO/Collected Amount (ml) O₂/(N₂ + O₂) Amount Battery 10.81 0.22 0.21 Battery 2 0.47 0.21 0.13 Battery 3 0.30 0.22 0.07Atmosphere 0.21 0.00* *A volume ratio of CO in the atmosphere is 10⁻⁷.Note ±5% is assumed to be within the range of measurement error.

The following is apparent from Table 5 and Table 6.

Although the battery 1 swelled after the discharge capacity test and thecollected amount of gas (corresponding to an amount of gas in a cell)was large compared with the battery 2 and the battery 3, the battery 1was not thought to generate oxygen since a volume ratio between oxygenand nitrogen (O₂/(N₂+O₂)) did not differ from ordinary atmosphericcomponents. Since in the battery 1, the volume ratio of CO gas(CO/Collected Amount) was increased, swelling of the battery 1 wasthought to result from the oxidation decomposition of the electrolytesolution at the positive electrode plate.

That is, even when the charge in the initial charge-discharge (firstcharge-discharge) was carried out up to 4.50 V as a battery voltage[4.60 V (vs. Li/Li⁺) as a positive electrode potential], the volumeratio of oxygen was not changed from that before the test, and oxygenwas not generated.

In the battery 2, the battery 3 in which the charge voltage in the firstcharge-discharge of the battery was set to 4.45 V, 4.40 V [4.55 V (vs.Li/Li⁺), 4.50 V (vs. Li/Li⁺) as a positive electrode potential] and thetest was performed, swelling of the battery was less than the battery 1charged at 4.50 V, and the collected amount of gas (corresponding to anamount of gas in a cell) was reduced. Further, the ratio between oxygenand nitrogen (O₂/(N₂+O₂)) was not changed, and the ratio of CO gas inthe collected gas (CO/Collected Amount) was reduced in conjunction withthe reduction of the positive electrode potential.

With respect to the discharge capacity at the time of charging at abattery voltage of 4.2 V [4.3 V (vs. Li/Li⁺) as a positive electrodepotential], in the battery 1, the battery 2 in which the charge in thefirst charge-discharge was carried out at 4.60 V (vs. Li/Li⁺), 4.55 V(vs. Li/Li⁺) as a maximum achieved potential of the positive electrode,both of the discharge capacities were large to the same extent, but inthe battery 3 in which the charge in the first charge-discharge wascarried out at 4.50 V (vs. Li/Li⁺), the discharge capacity is decreaseda little, and further when the charge in the first charge-discharge wascarried out at 4.30 V (vs. Li/Li⁺), the discharge capacity was extremelydecreased. Accordingly, when the positive active material of the presentinvention is used and the step including the initial charge-discharge(initial formation) is performed to manufacture a lithium secondarybattery, the maximum achieved potential of the positive electrode of thecharge in the initial charge-discharge is preferably set to 4.5 V ormore.

In view of FIG. 8 to FIG. 10 showing potential behavior at the initialcharge-discharge step of the batteries 1, 2 and 3, it has been foundthat a plateau potential of the positive active material of the presentinvention is around 4.5 V, and it has been confirmed that the activematerial of the present invention has the plateau potential in thecharge range and does not generate oxygen gas at the plateau potentialor more.

However, even when an active material not generating oxygen gas duringcharge is used, if the initial charge is performed to 4.6 V (vs. Li/Li⁺)or more as a maximum achieved potential of the positive electrode, gasis generated due to decomposition of an electrolyte solution.

Further, when the initial charge is performed at a potential less than4.5 V (vs. Li/Li⁺) as a maximum achieved potential of the positiveelectrode not undergoing a plateau potential as in the battery 4, thereis a problem that a discharge capacity at the time of charging to 4.2 Vas a battery voltage [4.3 V (vs. Li/Li⁺) as a positive electrodepotential] is small (refer to FIG. 11).

Therefore, the maximum achieved potential of the positive electrode ispreferably 4.5 V (vs. Li/Li⁺) or more and less than 4.6 V (vs. Li/Li⁺)even when the positive active material not generating oxygen gas duringcharge is used, and particularly, the maximum achieved potential ofabout 4.55 V (vs. Li/Li⁺) is thought to be an optimum initial formation(initial charge-discharge) condition.

DESCRIPTION OF REFERENCE SIGNS

-   1 lithium secondary battery-   2 electrode group-   3 positive electrode-   4 negative electrode-   5 separator-   6 battery case-   7 lid-   8 safety valve-   9 negative electrode terminal-   10 positive electrode lead-   11 negative electrode lead

INDUSTRIAL APPLICABILITY

The active material for a nonaqueous electrolyte secondary battery ofthe present invention can be effectively used for nonaqueous electrolytesecondary batteries of a power supply for electric vehicles, a powersupply for electronic equipment, and a power supply for electric powerstorage since the active material for a nonaqueous electrolyte secondarybattery has a large discharge capacity, and is superior incharge-discharge cycle performance, initial efficiency and high ratedischarge performance.

The invention claimed is:
 1. A positive active material for a nonaqueouselectrolyte secondary battery containing a lithium transition metalcomposite oxide which has a crystal structure of an α-NaFeO₂ type, isrepresented by a compositional formula Li_(1+α)Me_(1−α)O₂, wherein Me isa transition metal element including Co, Ni and Mn, and α>0, a molarratio Li/Me of Li to the transition metal element Me is 1.2 to 1.6, amolar ratio Co/Me of Co in the transition metal element Me is 0.02 to0.23, a molar ratio Mn/Me of Mn in the transition metal element Me is0.625 to 0.707, the lithium transition metal composite oxide is a singlephase attributed to a space group R3-m on an X-ray diffraction chartunder a condition in which the lithium transition metal composite oxideis electrochemically oxidized up to a potential of 5.0 V (vs. Li/Li⁺),and an oxygen position parameter of the lithium transition metalcomposite oxide is 0.260 or less, the oxygen position parameter beingdetermined in a state of a discharge end, by crystal structure analysisby a Rietveld method, using a space group R3-m as a crystal structuremodel based on an X-ray diffraction pattern.
 2. The positive activematerial for a nonaqueous electrolyte secondary battery according toclaim 1, wherein a BET specific surface area is 1.24 to 5.87 m²/g. 3.The positive active material for a nonaqueous electrolyte secondarybattery according to claim 1, wherein a tapped density is 1.25 g/cm³ ormore.
 4. The positive active material for a nonaqueous electrolytesecondary battery according to claim 1, wherein a ratio between thediffraction peak intensity I₍₀₀₃₎ of (003) line and the diffraction peakintensity I₍₁₁₄₎ of (114) line based on X-ray diffraction measurementbefore charge-discharge satisfies I₍₀₀₃₎/I₍₁₁₄₎≥1.20.
 5. The positiveactive material for a nonaqueous electrolyte secondary battery accordingto claim 1, wherein the lithium transition metal composite oxide isobtained by mixing/sintering a coprecipitated precursor of compounds ofthe transition metal elements including Co, Ni and Mn, and a lithiumcompound.
 6. A method of manufacturing the positive active material fora nonaqueous electrolyte secondary battery according to claim 1,comprising the steps of coprecipitating compounds of transition metalelements including Co, Ni and Mn in a solution to produce acoprecipitated precursor; and mixing/sintering the coprecipitatedprecursor and a lithium compound.
 7. The method of manufacturing apositive active material for a nonaqueous electrolyte secondary batteryaccording to claim 6, wherein a pH in the step of coprecipitatingcompounds of transition metal elements including Co, Ni and Mn in asolution to produce a coprecipitated precursor is 8.5 to 11.0.
 8. Themethod of manufacturing a positive active material for a nonaqueouselectrolyte secondary battery according to claim 6, wherein a sinteringtemperature in the step of mixing/sintering the coprecipitated precursorand a lithium compound is 800 to 940° C.
 9. An electrode for anonaqueous electrolyte secondary battery containing the positive activematerial for a nonaqueous electrolyte secondary battery according toclaim
 1. 10. A nonaqueous electrolyte secondary battery including theelectrode for a nonaqueous electrolyte secondary battery according toclaim
 9. 11. A nonaqueous electrolyte secondary battery comprising thepositive active material of claim 1, wherein a volume ratio of oxygen toa total of nitrogen and oxygen contained in a gas in the battery is 0.2to 0.25 under a condition in which the battery is charged so that amaximum potential of a positive electrode of the battery is 4.5 to 4.6 V(vs. Li/Li⁺).
 12. A positive active material for a nonaqueouselectrolyte secondary battery containing a lithium transition metalcomposite oxide which has a crystal structure of an α-NaFeO₂ type, isrepresented by a compositional formula Li_(1+α)Me_(1−α)O₂, wherein Me isa transition metal element including Co, Ni and Mn, and α>0, a molarratio Li/Me of Li to the transition metal element Me is of 1.2 to 1.6, amolar ratio Co/Me of Co in the transition metal element Me is 0.02 to0.23, a molar ratio Mn/Me of Mn in the transition metal element Me is0.625 to 0.707, the lithium transition metal composite oxide is a singlephase attributed to a space group R3-m on an X-ray diffraction chartunder a condition in which the lithium transition metal composite oxideis electrochemically oxidized up to a potential of 5.0 V (vs. Li/Li⁺),and a tapped density is 1.25 g/cm³ or more.
 13. A nonaqueous electrolytesecondary battery comprising the positive active material of claim 12,wherein a volume ratio of oxygen to a total of nitrogen and oxygencontained in a gas in the battery is 0.2 to 0.25 under a condition inwhich the battery is charged so that a maximum potential of a positiveelectrode of the battery is 4.5 to 4.6 V (vs. Li/Li⁺).
 14. A positiveactive material for a nonaqueous electrolyte secondary batterycontaining a lithium transition metal composite oxide which has acrystal structure of an α-NaFeO₂ type, is represented by a compositionalformula Li_(1+α)Me_(1−α)O₂, wherein Me is a transition metal elementincluding Co, Ni and Mn, and α>0, a molar ratio Li/Me of Li to thetransition metal element Me is of 1.2 to 1.6, a molar ratio Co/Me of Coin the transition metal element Me is 0.02 to 0.23, a molar ratio Mn/Meof Mn in the transition metal element Me is 0.625 to 0.707, and thelithium transition metal composite oxide is a single phase attributed toa space group R3-m on an X-ray diffraction chart under a condition inwhich the lithium transition metal composite oxide is electrochemicallyoxidized up to a potential of 5.0 V (vs. Li/Li⁺).
 15. The positiveactive material for a nonaqueous electrolyte secondary battery accordingto claim 14, wherein a BET specific surface area is 1.24 to 5.87 m²/g.16. A nonaqueous electrolyte secondary battery comprising the positiveactive material of claim 14, wherein a volume ratio of oxygen to a totalof nitrogen and oxygen contained in a gas in the battery is 0.2 to 0.25under a condition in which the battery is charged so that a maximumpotential of a positive electrode of the battery is 4.5 to 4.6 V (vs.Li/Li⁺).